Upgrade GitLab on geo-ash-10k-[primary|secondary].gogitlab.ml clusters to latest 12.7 nightly

gitlab!23545 (merged) fixed an Issue that is blocking Perform a planned failover using Geo HA but the fix is not yet deployed on the geo-ash-10k-[primary|secondary].gogitlab.ml clusters.

12.6.4+rnightly.141789.409adaed-0 is what geo-ash-10k-[primary|secondary].gogitlab.ml clusters are running atm. Let's upgrade GitLab to the latest 12.7 nightly package (gitlab-ee_12.7.6+rnightly.144861.438bf6df-0) in the hope it includes the fix 🙏

Related: gitlab#35734 (comment 289087582)