8.2
CS fixes
Add PR template and auto-close PR on subtree split repositories
[Notifier] Add push channel to notifier
Add support for sending to external user ids
Update license years (last time)
Use CPP where possible
Remove FQCN type hints on properties
Add call to backers in README files
Bump to php >= 8.4.1
Update XSD references in phpunit.xml.dist files