[TASK] Change php8.1 to 8.1.0RC6 and bump version
ReleaseCanditate 6 of PHP8.1 was released. Change base image for core-testing-php81 to 8.1.0RC6 and bump version.
RC6 also contains fix reported bug found through core unit tests https://bugs.php.net/bug.php?id=81441, and as such after bumping to RC6 the skipped testcase for ::1 can be reincluded.
Manual tested against core unit + functional tests with E_ALL active, TF master + fluid fork with branch lolli-13.