HHH-19584 add @CurrentTimestamp(allowMutation)

[gavinking]

[Please describe here what your change is about]

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https://hibernate.atlassian.net/browse/HHH-19584

[sebersole]

I'm confused why we are doing this. You even argue, rightly imo, against it on the Jira.

[sebersole]

Oh, scratch that; I see now - its just the contract, not the annotation