Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in block module

Thanks @divesh, we actually use a slightly different format to simplify things with namespace aliases so we can just write String::checkPlain() (without the extended prefix). I added instructions to the parent issue just now, that should hopefully make things clearer.

The code we want here is actually taking part of the patch at #2089465: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/modules/A-L but it could not be committed there due to the size of the patch - it needed to keep getting re-rolled as core changed.

seems fine.