bdns: deprecate DOH feature flag

[jsha]

Since the bdns unittests used a local DNS server via TCP, modify that server to instead speak DoH.

Fixes #8120

[github-actions[bot]]

@jsha, this PR appears to contain configuration and/or SQL schema changes. Please ensure that a corresponding deployment ticket has been filed with the new values.

[jsha]

Thanks, bot! This deprecates a flag that is already enabled; we'll do cleanup of deprecated configs from prod deploys in a later batch.

[beautifulentropy]

Excited to see all those clauses come out of the MPIC integration tests.