This commit changes the major version in the package name from v14 to
v15

Updating go.mod & go.sum with new module name v15

Update Makefile to bump major version to v15

Update the gitaly package name in the Makefile. Also update
gitaly-git2go-v14 -> gitaly-git2go-v15. We need to keep
gitaly-git2go-v14 for a release however, for zero downtime upgrades.
This pulls directly from a sha that is v14.

Update package name from v14->v15 for auth, client, cmd, internal packages

This commit changes the package name from v14 to v15 in go and proto
files in the internal, auth, client, cmd packages.

proto: Update major package number in package name

tools: Change major version number in package name from v14 to v15

gitaly-git2go: Change the package name from v14 to v15

update module updater for v15

Update the documentation for the module updater to reflect v15