[self-update] Restore global new-version notifications (#300)

[coisa]

Related Issue

Closes #300

Motivation / Context

• Global DevTools installs could stay silent even when a newer release was available because the version checker compared the Composer lookup result against a Symfony Process success constant that is not available in the supported runtime range.

Changes

• Replaced the version-check success comparison with Process::isSuccessful() after the Composer metadata lookup runs.
• Added ComposerVersionCheckerTest coverage for successful and failing Composer release lookups.
• Recorded the fix in CHANGELOG.md.

Verification

• composer dev-tools
• Focused command(s): composer dev-tools tests -- --filter='ComposerVersionCheckerTest'
• Manual verification: not rerun after the patch; the regression is covered by the new checker test and the full DevTools gate.

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The local .github/wiki modification was preserved outside this branch and is not part of this PR.

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-301/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-301/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-301/metrics/