Instructions to fix "invalid code integrity" pretty sparse

[dholbach]

Reading admin_manual/issues/code_signing.rst it's not quite clear

• why this can happen: how worried should I be? are bugs in the updater code (or whichever other parts of nextcloud relate to this) common?
• how to fix this exactly: the text merely says

  The solution is to upload the correct "index.php" and "version.php" files, and delete the "test.php" file.

  Here I'm wondering:
  • where do I get the correct files for my version of nextcloud from?
  • how do I confirm I got the right files?
  • do I need to stop/start the service?
  • ...