Fixes Out of memory issues when running checkstyle against large sourceSets

Context

With having ported the Checkstyle task to use the worker api and running in a separate java process (#20069) we see OOM problems
with our elasticsearch build when using the latest gradle master (7.5 snapshot)
This PR makes the heap size for checkstyle tasks configurable to not be forced to
rely on jdk defaults.

The property namings are similar to how memory settings are named in the Test task.

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