…system indices (elastic#49959)

This commit deprecates the creation of dot-prefixed index names (e.g.
.watches) unless they are either 1) a hidden index, or 2) registered by
a plugin that extends SystemIndexPlugin. This is the first step
towards more thorough protections for system indices.

This commit also modifies several plugins which use dot-prefixed indices
to register indices they own as system indices, and adds a plugin to
register .tasks as a system index.

…system indices (#49959)

This commit deprecates the creation of dot-prefixed index names (e.g.
.watches) unless they are either 1) a hidden index, or 2) registered by
a plugin that extends SystemIndexPlugin. This is the first step
towards more thorough protections for system indices.

This commit also modifies several plugins which use dot-prefixed indices
to register indices they own as system indices, and adds a plugin to
register .tasks as a system index.

This PR made the following changes:
1. Update to JDK 14
2. Hack to overstep potential ES 7.7 bug when installing plugins for test cluster
3. Upgrade gradle, nebula and jacoco versions
4. Change how we register rest endpoint handler as required by ES 7.7
5. Impelement SystemIndexPlugin and register the descriptor according to elastic/elasticsearch#49959
6. added new version of job scheduler zip

Testing done:
1. gralde build passes
2. Manually tested all public APIs

This PR made the following changes:
1. Update to JDK 14
2. Hack to overstep potential ES 7.7 bug when installing plugins for test cluster
3. Upgrade gradle, nebula and jacoco versions
4. Change how we register rest endpoint handler as required by ES 7.7
5. Impelement SystemIndexPlugin and register the descriptor according to elastic/elasticsearch#49959
6. added new version of job scheduler zip

Testing done:
1. gralde build passes
2. Manually tested all public APIs

Previously, we registered AD indices as system indices as AD indices' names start with a dot. ES deprecates the creation of dot-prefixed index names except for hidden and system indices (elastic/elasticsearch#49959). Starting 7.10, ES adds a dedicated thread pool for system index write operations. System index writes/reads have higher priority than user index writes/reads. For example, system index writes can be forced regardless of whether the current index pressure is high or not (https://github.com/elastic/elasticsearch/blob/242083a36e02496aae9214dc41b89372022e7076/server/src/main/java/org/elasticsearch/index/IndexingPressure.java#L62-L73). 

AD indices are not more important than other user indices. We don't want AD index reads/writes to impact user indices' reads/writes. This PR removes AD indices out of the system index list and marks them hidden indices instead.

This change does not impact created AD indices. They are still system indices. Newly created AD indices will be hidden instead of system indices. This change won't impact search/index API. To list hidden indices, one can use localhost:9200/_cat/indices?v&expand_wildcards=all

Testing done:
1. Ran backward-compatibility tests. After updating AD, old detectors run fine, and we can create/run new detectors. All public APIs still work.

Previously, we registered AD indices as system indices as AD indices' names start with a dot. ES deprecates the creation of dot-prefixed index names except for hidden and system indices (elastic/elasticsearch#49959). Starting 7.10, ES adds a dedicated thread pool for system index write operations. System index writes/reads have higher priority than user index writes/reads. For example, system index writes can be forced regardless of whether the current index pressure is high or not (https://github.com/elastic/elasticsearch/blob/242083a36e02496aae9214dc41b89372022e7076/server/src/main/java/org/elasticsearch/index/IndexingPressure.java#L62-L73). 

AD indices are not more important than other user indices. We don't want AD index reads/writes to impact user indices' reads/writes. This PR removes AD indices out of the system index list and marks them hidden indices instead.

This change does not impact created AD indices. They are still system indices. Newly created AD indices will be hidden instead of system indices. This change won't impact search/index API. To list hidden indices, one can use localhost:9200/_cat/indices?v&expand_wildcards=all

Testing done:
1. Ran backward-compatibility tests. After updating AD, old detectors run fine, and we can create/run new detectors. All public APIs still work.

Previously, we registered AD indices as system indices as AD indices' names start with a dot. ES deprecates the creation of dot-prefixed index names except for hidden and system indices (elastic/elasticsearch#49959). Starting 7.10, ES adds a dedicated thread pool for system index write operations. System index writes/reads have higher priority than user index writes/reads. For example, system index writes can be forced regardless of whether the current index pressure is high or not (https://github.com/elastic/elasticsearch/blob/242083a36e02496aae9214dc41b89372022e7076/server/src/main/java/org/elasticsearch/index/IndexingPressure.java#L62-L73).

AD indices are not more important than other user indices. We don't want AD index reads/writes to impact user indices' reads/writes. This PR removes AD indices out of the system index list and marks them hidden indices instead.

This change does not impact created AD indices. They are no longer system indices. Using indices metadata section in localhost:9200/_cluster/state?pretty output can confirm that (look for "system" key under each index ). Newly created AD indices will be hidden instead of system indices. This change won't impact search/index API. To list hidden indices, one can use localhost:9200/_cat/indices?v&expand_wildcards=all

Testing done:
* Ran backward-compatibility tests. After updating AD, old detectors run fine, and we can create/run new detectors. All public APIs still work.