chore: Override PHP default changelog sections

[devin-ai-integration]

Summary

Adds explicit changelog-sections configuration to release-please-config.json so that only feat and fix commits appear in release changelogs. This prevents chore/misc commits from triggering release entries, matching the fix applied in php-server-sdk#233.

The PHP release type in release-please is one of the few types that includes chore commits in changelogs by default. This override aligns the behavior with other LaunchDarkly SDKs.

Review & Testing Checklist for Human

• Verify the changelog-sections entries match the reference PR (only feat → Features and fix → Bug Fixes)
• Confirm no other commit types (e.g. perf, refactor) need to be included in changelogs for this repo

Notes

Requested by: @keelerm84
Link to Devin run

php-server-sdk-shared-tests was skipped as it has no release-please-config.json or release-please.yml.

[devin-ai-integration]

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