Fix PHPStan for previous major

I tried adding @phpstan-ignore-next-line lines, which worked for me in another module, but these were giving "no error to ignore" errors when testing against current versions.

I'm not very happy with the approach of adding ignore statements, since they could prevent genuine issues from being missed.

So I thought it would be good if it could be added for the previous major, which is the only time it's needed. Adding a before_script to insert the ignore lines.

However, after getting the tests to pass, I realised that deleting the attribute lines would achieve the same thing, but makes the script commands much simpler. So the updated .gitlab-ci.yml file includes a before script that runs for "phpstan (previous major)" only, that deletes the attribute lines.

I hoped to make this issue only about the attributes, but phpstan (next major) is required to pass, so I copied mike's change from ✨ Allow the possibility to specify which tags to be stripped when Strip HTML checkbox is ticked. Needs work to fix the deprecation issue.