go: Update gitlab-shell dependency to v14.8.0

Upgrade the gitlab-shell dependency to v14.8.0. This upgrade brings with it an incompatible change to the way the GitlabNetClient is constructed, which doesn't accept the SelfSigned parameter anymore.

Upstream has already gone through the deprecation phase of this parameter in the configuration, so in theory nobody should rely on it anymore. So consequentially, we should be fine to just drop it without any replacement ourselves.