[prep-release-3.1.0] Update version to 3.1.0
[prep-release-3.1.0] Prepare Changelog
[ticket/13208] Add security issues to the final changelog

PHPBB3-13208
Merge pull request #3059 from nickvergessen/ticket/13208

[ticket/13208] Add security issues to the final changelog
Merge branch 'prep-release-3.1.0' into develop-ascraeus

* prep-release-3.1.0:
  [ticket/13208] Add security issues to the final changelog
  [prep-release-3.1.0] Prepare Changelog
  [prep-release-3.1.0] Update version to 3.1.0