[redis] CC-2055: Upgrade PHP to v8.5

[andy1li]

Note

Upgrade PHP to 8.5 across configs/docs and add a new php-8.5 Dockerfile with required extensions.

• Runtime/Build:
  • Add dockerfiles/php-8.5.Dockerfile (Alpine 3.22), install linux-headers, enable pcntl and sockets.
  • Update buildpack to php-8.5 in compiled_starters/php/codecrafters.yml and solution ymls.
• Docs/Templates:
  • Bump required local PHP version references to php (8.5) in README.md files and starter_templates/php/config.yml.
  • Keep .codecrafters/compile.sh placeholders unchanged functionally.

^{Written by Cursor Bugbot for commit c2eb624. This will update automatically on new commits. Configure here.}

[linear]

CC-2055 Upgrade PHP to v8.5