add unset_versioning_override to WorkflowExecutionOptionsUpdatedEventAttributes

[carlydf]

READ BEFORE MERGING: All PRs require approval by both Server AND SDK teams before merging! This is why the number of required approvals is "2" and not "1"--two reviewers from the same team is NOT sufficient. If your PR is not approved by someone in BOTH teams, it may be summarily reverted.

What:

Add unset_versioning_override to WorkflowExecutionOptionsUpdatedEventAttributes

Why:

So that users of this event don't need to load VersioningOverride from mutable state every time they create this event.
This change was prompted because the event is now being used for non-version-override-related things, and I received feedback that it is inefficient / awkward / error-prone to have to load and pass in the current versioning override every time anyone writes to this event.

Now, a nil Versioning Override in this event means "no change" instead of "remove".
This reduces the chance that someone accidentally unsets an override in the future, and also is more efficient.
We've discussed this change internally in the server team and are ok with changing the meaning of this history event, because it is such a small change and the scope of impact is small (pre-release versioning users who have unset a versioning override and are building mutable state from that history).

Breaking changes?

Now, a nil Versioning Override in this event means "no change" instead of "remove".
If an event exists with the previous meaning and the mutable state is rebuilt, the Versioning Override would not be removed.
But the chance of that happening is very low.

temporalio/temporal#7091

[antlai-temporal]

This is backwards incompatible, but as described, it should not affect many workflows...