[prep-release-3.1.0-RC3] Update version to 3.1.0-RC3
[prep-release-3.1.0-RC3] Add migration for 3.1.0-RC3
[prep-release-3.1.0-RC3] Prepare Changelog
[prep-release-3.1.0-RC3] Add changelog for 3.1.0-RC3
[prep-release-3.1.0-RC3] Update phpBB Version for RC4 development