docs: update testing guide for PHPUnit 12

[imorland]

Summary

• Refresh docs/extend/testing.md example PHPUnit configs and test examples for PHPUnit 12.
• Add a PHPUnit section to docs/extend/update-2_0.md covering the 9.x → 12.x jump.

Why

Companion to flarum/framework#4585, which bumps `flarum/testing`'s PHPUnit constraint from `^11.0` to `^12.5.22` to clear GHSA-qrr6-mg7r-m243. The existing docs still show PHPUnit 9-era configs and `@test` docblock annotations — both broken on PHPUnit 10+, and silently broken on PHPUnit 12 (docblock annotations stop registering tests without warning).

testing.md

• Example `phpunit.integration.xml` / `phpunit.unit.xml` configs rewritten for the PHPUnit 12 schema (drop removed `convertErrorsToExceptions` / `convertNoticesToExceptions` / `convertWarningsToExceptions`, rename `backupStaticAttributes` → `backupStaticProperties`, drop the `` block, add schema declaration and `cacheDirectory`).
• All five `@test` docblock examples converted to `#[Test]` attributes with the matching `use PHPUnit\Framework\Attributes\Test;` import.
• Updated PHPUnit documentation links from `phpunit.readthedocs.io/en/9.3/` → `docs.phpunit.de/en/12.5/`.
• Added a migration callout under the example configs summarising the PHPUnit 12 requirement and pointing at the annotations guide.

update-2_0.md

• New PHPUnit (updated from ^9.5 to ^12.5) subsection under Infrastructure, matching the PHPStan/Larastan section's breaking/notable formatting.
• Calls out:
  • Annotations → attributes (with a complete before/after example, note about silent test non-discovery, and pointer to Rector's `ANNOTATIONS_TO_ATTRIBUTES` set).
  • Config schema attribute/element removals.
  • PHPUnit 12's new `AllowMockObjectsWithoutExpectations` notice (advisory, not breaking).

Test plan

• Rendered locally — no Docusaurus build errors, admonition renders.
• Reviewer spot-check that the example XML matches what core's tests/phpunit.integration.xml uses.