Eliminate the use of %VER in the RFCs in favor of the explicit version in the method IDs

[swcurran]

Sometime ago, in an attempt to DRY out the RFCs, it was decide that we would define the current protocol version at the top of the protocol, and use the string "%VER" in the message type strings (e.g. https://didcomm.org/issue-credential/%VER/propose-credential). The intention was to underline that the version in the message type strings must reflect the version of the protocol and that a version associated with an individual message is not applicable. All messages within a protocol must have the same version in the message type string. By using %VER we could save having to be sure that every message type string was updated when the version of the protocol was bumped.

The problem with that idea was that the convention in the use of "%VER" is not obvious to those not deep in the community, and we've seen lots of confusion about what "%VER" means. In looking up %VER across the entire repository, I don't find anywhere the term is explained. I've seen code that has the literal string "%VER" in the code.

I suggest that we eliminate the use of that convention, replacing all instances of "%VER" in the message type strings with the correct protocol version for the protocol. I would be happy to do the PR for that -- including making sure that the "Protocols" RFC is clear that the version in a message type string is the protocol version.