improve handling of invalid multiregion deploys vs OSS

[tgross]

If you run a multiregion job against an OSS region, the current behavior is that the deployment gets stuck in pending and you need to fix your jobspec to continue. We didn't have this error-out because we were concerned about what would happen in a ENT cluster was downgraded to OSS, leaving jobs in the state store with multiregion stanzas.

We need to:

• improve the messaging to the operator around this case (at least return a warning!)
• document the behavior
• see if we can come up with way to reject the job safely without breaking the downgraded cluster use case.

[tgross]

It looks like the ENT->OSS downgrade case isn't something we support, so that simplifies the problem quite a bit. We'll pull the (*Multiregion) Validate() method into ENT and just have it return an error if non-nil in OSS.

[tgross]

Will ship in 0.12.4

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.