[CI] Collect and report deprecation statistics and details

Problem/Motivation

Follow up of #3523383: Tidy up and tighten deprecation skips.

The DeprecationHandler class was implemented as a replacement of Symfony's PHPUnit-bridge component, and uses initialization logic that is now outdated by more recent PHPUnit developments.

Proposed resolution

In this issue, we will

1. convert the DeprecationHandler class into a proper PHPUnit 'extension', using the bootstrap logic introduced in PHPUnit 10
2. introduce proper extension parameters in the phpunit.xml to configure the extension, and a new environment variable that could override the xml configuration at runtime in CI pipelines
3. make the legacy SYMFONY_DEPRECATION_HELPER variable redundant (still allowing its usage at least for now)
4. introduce a mechanism to report on each CI job, and finally consolidate at the end of a pipeline execution, statistics about: a) are the deprecation ignore patterns in the .deprecation-ignore.txt file actually necessary and b) are tests marked with the #[IgnoreDeprecations] attribute actually triggering any deprecation. So that with those information we can cleanup tests from irrelevant settings (that could also silently ignore other unexpected deprecations).

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet