Rename PHPUnit 10 guide to PHPUnit Upgrade and add PHPUnit 11/12 sections

[dereuromark]

Summary

• Renamed phpunit10.md to phpunit-upgrade.md and updated nav title to "PHPUnit Upgrade"
• Restructured existing PHPUnit 9→10 content under its own section
• Added PHPUnit 10→11 migration section (annotations deprecated, stub expectations, abstract class mocks, test class naming)
• Added PHPUnit 11→12 migration section (annotations removed, abstract class mocks removed, stub expectations removed)
• Added Rector section for automated migration
• Updated all internal references (TOC, migration guides, 5.0 migration guide) in both EN and JA

Closes #8172

[LordSimal]

i doubt those _ANALYSIS_SUMMARY.md files need to be present in the repo as they seem to be planning files from your AI @dereuromark

[dereuromark]

They are already gone again, a squash merge will merge this cleanly IMO.