Get rid of module version in Gitaly

Currently we have a Gitaly module version (v15) appended to the package name. This is to version our Go module for backwards compatibility reasons. This mechanism is generally used for APIs.

However, this ends up creating more burden than is useful for us because each major release we have to upgrade everything as we did in v15, and it doesn't really give us much benefit.

Last time around, we tried to get rid of this but ran into a couple of issues:

• Tagged major versions will break with go get (go enforces major package version in the package name past v1)
• Go get without a version will end up pulling in v14 since it's the latest

In the end we just ended up bumping the version number due to time pressure.

We should discuss and find a way to get rid of this module version if we can in 16.0