* don't timestamp active log file

* website: update log_file default value

* changelog: add entry for #11070

* website: add upgrade instructions for log_file in v1.14 and v1.2.0

* don't timestamp active log file

* website: update log_file default value

* changelog: add entry for #11070

* website: add upgrade instructions for log_file in v1.14 and v1.2.0

This patch updates the file_sink to *not* append the timestamp on the
current log file. The timestamp is only appended when the file is
rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061
- Referenced in Consul's documentation:
  https://www.consul.io/docs/enterprise/audit-logging

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

While this is a *backward incompatible change* I think it's worth it for
usability and matching Consul's documentation. Nomad's documentation can
be fixed and a notice placed in the Upgrade Guide.

Backward compatible version of #62 -- This version does *not* change the
default behavior. Users must opt in to new behavior.

This patch updates the file sink to allow *not* appending the timestamp
on the current log file. In this mode the timestamp is only appended
when the file is rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061
- Referenced in Consul's documentation: https://www.consul.io/docs/enterprise/audit-logging

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

While this is a *backward incompatible change* I think it's worth it for
usability and matching Consul's documentation. Nomad's documentation can
be fixed and a notice placed in the Upgrade Guide.

In a bid to try to win your support I labeled this PR a bug despite the
tests and comments being very clear the old behavior (always append a
timestamp) *was* intentional.

Backward compatible version of #62 -- This version does *not* change the
default behavior. Users must opt in to new behavior.

This patch updates the file sink to allow *not* appending the timestamp
on the current log file. In this mode the timestamp is only appended
when the file is rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

* allow not appending timestamp to current log file

Backward compatible version of #62 -- This version does *not* change the
default behavior. Users must opt in to new behavior.

This patch updates the file sink to allow *not* appending the timestamp
on the current log file. In this mode the timestamp is only appended
when the file is rotated.

This matches the behavior:

- Implemented in Nomad v1.1.4 with hashicorp/nomad#11070
- Requested in hashicorp/nomad#11061

> The following example configures a destination called "My Sink" which
> stores audit events at the file `/tmp/audit.json`.

Nomad's documentation is a little vague but implies the prior behavior
of always appending a timestamp.

Also fixed one flaky test:

* time may be either the same of after

While Go compares time using a monotonic clock that only prevents time from
moving *backward,* it does *not* guarantee time will move forward
between subsequent calls to `time.Now()`. The After assertion failed in
a test run on Github because it is missing the `Equal` comparison on the
preceeding assertion.