Remove validation of "Invalid response code" for administrators

MR has my proposed solution which is just check if it's numeric.

I'm wondering why \Symfony\Component\HttpFoundation\Response::$statusTexts is not being used instead of \Drupal\linkchecker\LinkCheckerResponseCodes::$responseCodes

The Symfony response class has a 418 status code as an allowed response code so using it should resolve the problem of this issue.

@quadrexdev good to know there is duplicate, maybe a separate issue to replace that? I just created a separate MR for the missing 308 issue that might be a better place for that fix ✨ Status code 308 for Permanent Redirect Needs review

I am hoping with this change that we don't even validate it so we can allow non-standard codes to be ignored like LinkedIn's 999 for example.

Retitled to better indicate the proposed solution's intent

@joelpittet Yep, sounds reasonable to work on replacement in #2958196