Update the Upgrade GitLab Runner link to make use of the new documentation format
Proposal
In gitlab-runner!4780 (merged), we updated the documentation layout on the GitLab Runner Install page so we have a section dedicated to Upgrades. That means we can now update the link in the upgrade popover Upgrade GitLab Runner
to link directly to this new section.
Update Upgrade GitLab Runner
link from https://docs.gitlab.com/runner/install/ to https://docs.gitlab.com/runner/install/#upgrading-gitlab-runner.