go: Bump major version to v14

While gitlab-shell currently has a major version of v14, the module path it exposes is not using that major version like it is required by the Go standard. This makes it impossible for dependents to import gitlab-shell as a dependency without using a commit as version.

Fix this by changing the module path of gitlab-shell to instead be gitlab.com/gitlab-org/gitlab-shell/v14 and adjust all imports accordingly.

Changelog: fixed

Fixes #593 (closed).