`/_matrix/key/v2/server` says it's signed by "the verify keys", but which ones from the list should be used?

**Link to problem area**: https://spec.matrix.org/v1.9/server-server-api/#get_matrixkeyv2server

**Issue**
I believe *all* of the `verify_keys` need to sign the object, but it's not impossible that we only require one.

Related: https://github.com/matrix-org/matrix-spec/issues/510

Start with the GET /_matrix/key/v2/server section linked in the issue, then read the related matrix-spec issue #510 for the discussion about verify_keys. Done means the specification clearly states which verify_keys must sign the object and the ambiguity in this section is resolved.

Written by the indexing model from the issue text.