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Drop init container annotations during conversion
https://github.com/kubernetes/kubernetes/pull/51816/files#diff-0fa94eafdf1c7cd2171c836b53fd6323 removed special handling of init container annotations in API conversion in 1.8
However, older clients (like 1.7 kubectl) still performed that handling in some paths, which would make an object round-tripped through kubectl show up with additional annotations.
Those additions would get flagged as disallowed mutations in some objects.
This change strips init-container annotations during conversion so that old clients sending init container annotations (which are inert in 1.8+) don't trigger validation errors around immutable fields.
Fixes#54816
```release-note
Restores the ability of older clients to delete and scale jobs with initContainers
```
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