Move all manager task validation to validating webhook

[rzetelskik]

What should the feature do?

Currently only some of the manager task fields are validated by the validating webhook, while the others are validated at runtime. We should move all validation to the validating webhook, instead of failing during scheduling. Unit tests should also cover the validation.

/priority important-longterm /kind feature /assign

What is the use case behind this feature?

https://github.com/scylladb/scylla-operator/pull/1851#discussion_r1539259781

Anything else we need to know?

No response