Do we need a community update to transition to new DIDComm message mime-type?

This #571 changed the mime-type for DIDComm messages, and the feeling was that we didn't need to change the frameworks to make this happen.

We did an interop test in the https://github.com/hyperledger/aries-agent-test-harness between ACA-Py and AF-Go and we have a break on mime-type. AF-Go is using `application/didcomm-envelope-enc` and ACA-Py is using `application/ssi-agent-wire` and neither is handling the other. We can fix that, by changing both the accept both, but where does that leave Aries Framework .NET and other frameworks?

Do we need to do a community update so that:

* all change to handling both and send out the old for awhile
* then transition to all sending out the new, and
* then change all to drop the first.

We can use Aries Agent Test Harness to find out what Aries Framework .NET is doing and I'll report back here.

@esplinr -- do you know what Evernym is doing?