build: Remove global per-test JVM heap cap (#5671)

* build: Remove global per-test JVM heap cap

The minHeapSize/maxHeapSize cap in the root build.gradle.kts was applied
to every module's test task. Most modules do not need it, so remove it
and let tests use the JVM defaults. If a specific module turns out to
require a larger heap, the cap can be re-added to that module only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* build: Restore per-test heap cap for sentry-android-core

CI showed :sentry-android-core:testReleaseUnitTest fails with
OutOfMemoryError (Robolectric loading the android-all jar) once the
global cap is removed. Restore the 256m/2g cap for this module only,
where it is actually needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* build: Drop stale comment about root build.gradle.kts

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>