Added Troubleshooting step if GPG key of GitLab has changed

What does this MR do and why?

This MR adds a section to the troubleshooting page when a GitLab update fails due to an updated GPG key.

Screenshots or screen recordings

Error An error occurred during the signature verification when running apt-get update

To update the GPG key of the GitLab packages server run:

curl --silent "https://packages.gitlab.com/gpg.key" | apt-key add -
apt-get update

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.