Move version out of SDK.

[ncabatoff]

The goal of this work is to simplify our release process so that we don't have to update the version in sdk, then update go.mod to pull in the new sdk version. Instead we put the version into the vault package itself.

The reason it was in the sdk was I think to allow plugins to have access to it. But that was wrongheaded: external plugins shouldn't be reporting the vault version they were built with as the vault version of the cluster, and builtin plugins are built with vault and thus don't need to go through the sdk to get the vault version. I've tried to make it so that regardless of the plugin type, they can get the correct vault version of the cluster to put into their user-agent string. This requires that they use the useragent.PluginString method instead of useragent.String. Followup work to modify plugins we own to do that is still needed.

[kalafut]

Seems about right (aside from needing a run of gofumpt)

[calvn]

It looks like vault-plugin-auth-azure uses sdk/useragent.