fix: make cluster reference immutable on Database, Pooler, Publication, Subscription and ScheduledBackup (#10743)

Add a CEL `self == oldSelf` validation rule to the `cluster` field of
the Database, Pooler, Publication, Subscription and ScheduledBackup
resources so that the target cluster cannot be changed after the
resource is created.

Repointing these objects at a different cluster has no well-defined
semantics and previously left the controllers in an inconsistent state;
rejecting the update at the API server makes the constraint explicit to
users.

Regenerate the affected CRD manifests and document the new constraint in
the corresponding user guide sections.

Signed-off-by: Leonardo Cecchi <leonardo.cecchi@enterprisedb.com>
Signed-off-by: Armando Ruocco <armando.ruocco@enterprisedb.com>
Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>
Co-authored-by: Armando Ruocco <armando.ruocco@enterprisedb.com>
Co-authored-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>