Do not start Xdebug automatically, use sessionkey instead

[delawski]

This change prevents console errors when running PHPUnit tests as described in #1508 (comment)

Xdebug will no longer be automatically enabled whenever a request comes in. Instead, a request should contain a sessionkey added by a browser extension like Xdebug helper.

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).

[tharsheblows]

@delawski I use xdebug with phpunit a lot 😅 -- but I do think we should sort the underlying issue of "don't show warnings or deprecations before the unit tests" if possible. This isn't an issue with running the tests in php 7.4 but is in php 8.2 even with the correct phpunit.xml settings; they're still before the tests for some reason.

Can we address this in this issue? #1278

[delawski]

This is superseded by #1511.