Apply constructor property promotion via Rector.

[shadyvb]

XWPENG-47 (sub-PR 4 of the typed-properties stack; base: ticket/XWPENG-47-typed-props-alerts).

Applies PHP 8 constructor property promotion to seventeen core service classes via Rector's ClassPropertyAssignToConstructorPromotionRector. Each class that previously declared a property and assigned it in __construct() now promotes the parameter directly — for example public function __construct( public Plugin $plugin ) — removing redundant property declarations and $this->prop = $prop boilerplate.

Classes updated: Abilities, Ability (protected), Admin, Alert, Alerts, Alerts_List, Alert_Trigger, Alert_Type, Connectors, DB, Export, Install, List_Table, Live_Update, Log, Network, Settings.

rector.php registers the promotion rule with RENAME_PROPERTY => false so existing property names are preserved.

No runtime behavior changes — this is a readability and modernization refactor only.

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 — internal refactor, no user-facing change.
• New: N/A.

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.

Change [ ] to [x] to mark the items as done.