Backup: Write helper script to ABSPATH by default, just like we did b…

…efore (#35508)

* Write helper script to ABSPATH, just like we did before

Writing to "wp-content/" has caused some issues, as some hosts don't allow PHP code from "wp-content/" to be accessed directly.

* Bump class namespace version

Automattic/jetpack#34739

* changelog

* Make the test expect the right order of install locations

* Fix up project versions

* Add proxy `Package_Version` classes with different namespaces

* Revert "Add proxy `Package_Version` classes with different namespaces"

This reverts commit abd558f3cb0d8145c2091840343a17b0d080ee1c.

* Don't use namespace versioning in Package_Version, add <...>\V0001\Package_Version proxy

* Use correct path to Package_Version in the test

* Fix package name in a test

* Move test to Automattic\Jetpack\Backup namespace

* Move Package_Version's compatibility class to separate file

* Add (possibly) missing import

* Apply the same behavior to Automattic\Jetpack\Transport_Helper\Package_Version too

* Fix up project/package versions again

* Fix transport-helper's actions.php to point to the right Package_Version

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/7823601613