Convert two switch statements to match expressions.

[shadyvb]

Fixes XWPENG-47.

Part of the XWPENG-47 Rector modernization stack. This PR targets ticket/XWPENG-47-constructor-promotion and converts the two remaining eligible switch statements in the Rector scan paths to PHP 8.0 match expressions. Behavior is unchanged; the refactor uses expression-style branching that is easier to read and aligns with the PHP 8.2 baseline.

Summary

• Ability_Get_Alerts::execute() — maps the status input (enabled, disabled, or default/any) to the alert status filter array via match.
• List_Table::get_column_excluded_setting_key() — maps list-table column slugs to their exclusion-setting keys via match.
• rector.php — registers ChangeSwitchToMatchRector so composer lint-rector reflects the transformations present in this branch.

No new PHPUnit coverage is required: both methods preserve identical return values for every branch.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: N/A — no user-facing bug fix in this stacked refactor.
• New: N/A — no new feature in this stacked refactor.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.