[FEATURE] ext:saltedpasswords is required and enabled in backend

This patch adds saltedpasswords as a required system extension,
forcing salted hashes to be enabled for backend authentication.
The install tool checks settings and adapts them if needed.
Rsaauth is added to the list of recommended extensions
in the install tool extension installation upgrade wizard.

Resolves: #51352
Releases: 6.2
Change-Id: Ifd3b9f195101bcdb083b3bf9db4a74c812f0a709
Reviewed-on: https://review.typo3.org/23333
Reviewed-by: Kai Ole Hartwig
Tested-by: Kai Ole Hartwig
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn