Show explicit message for "Re-Check version" if installed version is still up to date

When you call the function in question, you don't get any explicit feedback if the phpBB version is still current and if there were no errors during the version check.

This can be fixed with a small addition that generates an additional template variable, which can then be queried in the template to display an explicit message, see screenshot. Everything else that is needed is already there, including the appropriate language variable.

Reference: https://www.phpbb.de/community/viewtopic.php?t=247242

PR follows