[ticket/12326] Do not add deleted files to the list of changed files

PHPBB3-12326
[ticket/12326] Correctly format update/index.php and remove php closing tag

PHPBB3-12326
Merge remote-tracking branch 'nickvergessen/ticket/12326' into develop-ascraeus

* nickvergessen/ticket/12326:
  [ticket/12326] Correctly format update/index.php and remove php closing tag
  [ticket/12326] Do not add deleted files to the list of changed files
[prep-release-3.1.0-b2] Update version to 3.1.0-b2
[prep-release-3.1.0-b2] Add migration for 3.1.0-b2
[prep-release-3.1.0-b2] Prepare Changelog
[prep-release-3.1.0-b2] Add changelog for 3.1.0-b2