MSC3861: m.3pid_changes capability should be disabled

[MatMaul]

Fixes #16128.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[hughns]

FWIW, the approach that has been used elsewhere for MSC3861 is to error if the admin has tried to explicitly enable a feature that is not compatible with delegated auth. i.e. https://github.com/matrix-org/synapse/blob/develop/synapse/config/experimental.py#L151

I was intended to write a PR for this myself which did:

1. Change the default for enable_3pid_changes to be False if experimental MSC3861 is enabled: at https://github.com/matrix-org/synapse/blob/develop/synapse/config/registration.py#L136
2. Added a runtime check at https://github.com/matrix-org/synapse/blob/develop/synapse/config/experimental.py#L151
3. Unit tests to the runtime config error fires when misconfigured: https://github.com/matrix-org/synapse/blob/develop/tests/config/test_oauth_delegation.py#L56

[clokep]

Per @hughns's review.

PR Updated

[hughns]

LGTM 👍

[clokep]

I combined the news file with #16127.