feat: make linked-versions pull request title configurable

[Leundai]

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Summary

This PR makes the linked-versions plugin's pull request title configurable via the group-pull-request-title-pattern option. Previously, the title was hardcoded to chore${scope}: release ${groupName} ${version}. Now users can customize it while maintaining backward compatibility.

Changes

• Added groupPullRequestTitlePattern option to plugin factory and manifest
• Updated linked-versions plugin to use configured pattern, defaulting to chore${scope}: release ${groupName} libraries for backward compatibility
• The ${component} placeholder in the pattern is replaced with the plugin's groupName
• Added documentation in customizing.md and manifest-releaser.md
• Added 3 new tests covering default behavior and custom patterns

Human Review Checklist

• Verify backward compatibility: default title should include "libraries" suffix (not ${version})
• Confirm ${component} placeholder replacement works correctly in linked-versions.ts:183-187
• Check that the default pattern change from ${version} to libraries is intentional

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This is for our internal update here

created an upstream PR too
googleapis#2617

Fixes #<issue_number_goes_here> 🦕