Remove Gitaly prefix to Praefect server version

Example from before:

~/src/gitlab.com/gitlab-org/gitaly master
$ ./praefect --version
Gitaly, version 1.61.0-11-gceb7f598

Example form after this change:

~/src/gitlab.com/gitlab-org/gitaly zj-praefect-server-version
$ ./praefect --version
Praefect, version 1.61.0-10-g41789fd1