baseline-java-versions: Remove setting sourceCompatibility in task action

[CRogers]

Before this PR

In #2136, we forced setting the source compatibility in a task action to work around a bug in Gradle where it would add the --release flag unnecessarily, which meant we could not use --add-exports or --add-opens.

This has a couple of problems:

1. Since it access mutable task state inside a task action, it's not good Gradle:
   1. The task cannot be run in parallel with other tasks in the same project
   2. Not configuration cache compatible
2. It prevents people from setting their own sourceCompatibility and (targetCompatibility if it is less than the sourceCompatibility). I want to do this for palantir-java-format to support running in older compiler versions.

This bug was fixed in Gradle 8, so we don't need to do this any more.

After this PR

==COMMIT_MSG==
For baseline-java-versions (for Gradle >=8), do not explicitly set the sourceCompatibility of Java compilations in a task action before the task runs.
==COMMIT_MSG==

Possible downsides?

[changelog-app]

Generate changelog in changelog/@unreleased

What do the change types mean?

• feature: A new feature of the service.
• improvement: An incremental improvement in the functionality or operation of the service.
• fix: Remedies the incorrect behaviour of a component of the service in a backwards-compatible way.
• break: Has the potential to break consumers of this service's API, inclusive of both Palantir services
  and external consumers of the service's API (e.g. customer-written software or integrations).
• deprecation: Advertises the intention to remove service functionality without any change to the
  operation of the service itself.
• manualTask: Requires the possibility of manual intervention (running a script, eyeballing configuration,
  performing database surgery, ...) at the time of upgrade for it to succeed.
• migration: A fully automatic upgrade migration task with no engineer input required.

Note: only one type should be chosen.

How are new versions calculated?

• ❗The break and manual task changelog types will result in a major release!
• 🐛 The fix changelog type will result in a minor release in most cases, and a patch release version for patch branches. This behaviour is configurable in autorelease.
• ✨ All others will result in a minor version release.

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

For baseline-java-versions (for Gradle >=8), do not explicitly set the sourceCompatibility of Java compilations in a task action before the task runs.

Check the box to generate changelog(s)

• Generate changelog entry

[autorelease3]

Released 6.19.0