Require previousKeyId and previousKey to be set together

What does this MR do?

• Require previousKeyId and previousKey to be set together.

• previousKeyId without previousKey already failed to render, but the reverse silently dropped the previous key from the seal config, so an operator who set the path and forgot the id ended up with a pod that could not unseal.

• Blanking the previousKey default is what makes the check possible: a non-empty path with an empty id was the shipped default, so a symmetric check would fail to render the chart on its own values. It also lines this chart up with the GitLab chart, which defaults both to empty and documents them as a pair to set and remove together.

• Raised in review of gitlab-org/charts/gitlab!5186

• Relates gitlab-org/gitlab#603143

Author's Checklist

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Expected

• Test plan indicating conditions for success has been posted and passes.
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