Remove support for PHPUnit 10

Following up from 🐛 Can't update to 11.2.0 Active here: with PHP10 support removed, consider restoring the core-recommended composer.json constraint on sebastian/diff.

Not sure about #2. If we readd here, we may need to remove again in a few months while adjusting for PHPUnit 12.

I asked in #maintainers in Slack, and those who responded were largely unaware that we had moved to PHPUnit 11 even, but their code was still working fine, or they had had to make minimal changes. I think we should give it a few more days but it seems so far that dropping PHPUnit 10 support won't be a problem even though it is technically a BC break.

I will credit those who contributed to the discussion.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

rebased