Modernize the version requirements maintenance workflow

[Abdalsalaam]

Changes proposed in this Pull Request:

Replaces the outdated update-requires-headers.yml workflow with maintenance-update-version-requirements.yml ("Maintenance: Update version requirements"):

• Bumps Tested up to when a WordPress RC is available, and WC tested up to once beta 2 of the next WooCommerce minor is out.
• Sets Requires at least to WP stable - 1 and WC requires at least to WC stable - 2. Requires PHP mirrors WooCommerce core. All updates are upgrade-only, never downgrades.
• Updates the headers in the main plugin file, readme.txt, and README.md wherever they exist, and adds matching changelog entries to changelog.txt and the readme changelogs, reusing the unreleased section when one exists (without duplicating entries) or creating one for the next patch version.
• Auto-detects the main plugin file and the default branch, so the same workflow file is shared across all extension repos with no per-repo configuration.
• Skips the run when a version update PR is already open (including PRs from the old workflow).
• No more WooCommerce zip download; versions come from the WordPress.org and GitHub APIs.

The generated PRs request reviews from the team and serve as the reminder to test the plugin against the upcoming WordPress/WooCommerce versions before merging.

Same change as woocommerce/woocommerce-shipping-usps#585, verified end-to-end with manual runs before rollout.

Backward compatibility impact:

No BC impact. CI-only change, no plugin code touched.

How to test the changes in this Pull Request:

1. Merge, then trigger "Maintenance: Update version requirements" from the Actions tab.
2. Check the PR it opens: bumped headers and changelog entries, reviewers requested.
3. Trigger the workflow again and confirm the run skips because an update PR is already open.

Note: PRs created by the workflow use the default GITHUB_TOKEN, so CI does not start on them automatically. Close and reopen such a PR to trigger CI.

Changelog entry

None, developer tooling change only.