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.