Post 7.1 branching changes

[t-hamano]

This includes all the changes required post-branching for trunk:

• Bump the version in version.php, composer.json, package.json, and package-lock.json to 7.2-alpha.
• Add 7.1.x to the list of versions receiving security updates in SECURITY.md.
• Add the 7.1 branch to the test-old-branches.yml workflow matrices, and update CURRENTLY_SUPPORTED_BRANCH.
• Add 7.1-RC2 to the upgrade testing workflow matrices. Since these workflows test tagged versions available through WP-CLI, the pre-release version needs to be used until 7.1 is released.
• Add 7-2 keys to .version-support-mysql.json and .version-support-php.json.

Note: the SVN revision in version.php (7.2-alpha-63162-src) is a placeholder and will need to match the actual revision at commit time.

Trac ticket: https://core.trac.wordpress.org/ticket/65844

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Applying the mechanical post-branching version/matrix updates following the handbook and the previous 7.0 branching PR (#11376); reviewed and verified by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props wildworks, joedolson.

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