Remove deprecated singular CRD entries

[Joibel]

As part of 3.6 we deprecated 4 features:

• podPriority doesn't work any modern kubernetes and has been deprecated for a while
• schedule is replaced by schedules in cron workflows - this is a trivial update to make to the yaml
• mutex and semaphore are replaced by the pluralised version - this is a trivial update to make to the yaml

Removing legacy code reduces maintainer burden and the chance for errors.