External Libraries: Update jQuery Migrate to 3.6.0

[rootzero00]

Trac ticket: https://core.trac.wordpress.org/ticket/51812

Summary

Update the bundled jQuery Migrate library from 3.4.1 to 3.6.0 while keeping jQuery Core at 3.7.1.

The change updates:

• src/js/_enqueues/vendor/jquery/jquery-migrate.js
• src/js/_enqueues/vendor/jquery/jquery-migrate.min.js
• the registered jQuery Migrate version in src/wp-includes/script-loader.php

Validation

The candidate was validated against current WordPress runk.

Completed validation included:

• WordPress JavaScript build and pre-commit-equivalent checks
• TypeScript and compiled QUnit checks
• PHPCS and PHPCBF no-op verification
• PHPStan
• the full seven-target PHPUnit matrix
• frontend and admin runtime checks
• runtime verification of jQuery 3.7.1 and jQuery Migrate 3.6.0
• verification that jQuery loads before jQuery Migrate
• development Migrate warning capture and separation from JavaScript runtime errors
• final refresh and revalidation against current runk

No unresolved candidate-attributable regression was found.

AI assistance

AI assistance was used during investigation, validation design, and preparation of this contribution. All resulting changes were reviewed and validated by the contributor.

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This pull request is for code review against the existing WordPress Core Trac ticket. It is not intended to be merged directly on GitHub.

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