Add ci_success job to simplify branch protection

[jeffreyparker]

Summary

• Add a single CI Success job that depends on all matrix jobs passing
• This creates one status check instead of individual checks for each PHP version

Benefits

• Branch protection settings will only need to require the single CI Success check
• Adding or removing PHP versions from the matrix won't require updating branch protection rules
• Simplifies maintenance when deprecating old PHP versions

Test plan

• Verify CI passes on this PR
• After merge, update branch protection to require only CI Success instead of individual PHP version checks