Message: Disallow unspecified ClientMismatchStrategy

[akshaymankar]

Checklist

• Title of this PR explains the impact of the change.
• The description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• The CHANGELOG.md file in the Unreleased section has been updated to explain the change which will be included in the release notes.
• If a component uses a new or changed internal endpoint of another component, this is mentioned in the CHANGELOG.md.
• If this PR creates a new endpoint, or adds a new configuration flag, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.

[jschaul]

Looks good, see comment

[jschaul]

Is this backwards compatible with old clients who use protobuf on the old endpoint? Or is the unqualified endpoint making use indirectly of this function, too?

[akshaymankar]

The unqualified endpoint is not using this at all.