Upgrade/Install: Remove accidentally shipped RTC files on upgrade

[itzmekhokan]

Summary

• Adds real-time collaboration files to the $_old_files array in update-core.php so they are removed during core upgrades.
• Addresses #65325, where RTC files removed in [62334] were unintentionally included in the WordPress 7.0 release package due to a build server issue.
• The existing verify:old-files Grunt task will also fail builds if these files are present in the build/ directory, preventing recurrence.

Context

Real-time collaboration was removed from core in #65205 (62334, 62337, 62338 on the 7.0 branch). However, the files still shipped in wordpress-7.0.zip and the WordPress/WordPress 7.0 tag. The impact is mostly benign since the files are not loaded, but they should be cleaned up on upgrade and excluded from future builds.

Test plan

• Verify php -l src/wp-admin/includes/update-core.php passes
• Verify PHPCS reports no errors on the changed file
• Confirm build/wp-includes/collaboration/ does not exist locally
• Run npx grunt verify:old-files as part of a full build in CI
• Confirm upgrading from a 7.0.0 install with RTC files to 7.0.1 removes:
  • wp-includes/collaboration.php
  • wp-includes/collaboration/class-wp-http-polling-sync-server.php
  • wp-includes/collaboration/class-wp-sync-post-meta-storage.php
  • wp-includes/collaboration/interface-wp-sync-storage.php

Backport

This change should be backported to the 7.0 branch for the 7.0.1 milestone.

Trac ticket: https://core.trac.wordpress.org/ticket/65325

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