Add final Runner upgrade step to CI/CD section

Co-authored-by: Kati Paizee <kpaizee@gitlab.com>
Co-authored-by: Suzanne Selhorn <sselhorn@gitlab.com>

doc/update/index.md

@@ -250,6 +250,8 @@ To address the above two scenario's, it is advised to do the following prior to
 1. Pause your runners.
 1. Wait until all jobs are finished.
 1. Upgrade GitLab.
+1. [Update GitLab Runner](https://docs.gitlab.com/runner/install/index.html) to the same version
+   as your GitLab version. Both versions [should be the same](https://docs.gitlab.com/runner/#gitlab-runner-versions).
 
 ## Checking for pending Advanced Search migrations **(PREMIUM SELF)**