GitLab: updates to v12.8.1 and adds some custom alerting, plus tweaks
and fixes

Thanos: updates to v0.10.1 or newer, uses Thanos Store's experimental
features to reduce memory impact and OOM errors. Importantly, also
connects the SFO cluster to Thanos Querier through VPN + kubectl proxying.

Node termination handling: adds the GKE node termination handler on
top of estafette-preemptible-killer, since GKE-induced node
terminations were still being observed.

NGINX ingress: updates to v0.29.0 and adds some global configuration
of proxy_next_upstream and proxy_buffering to try to deal with
upstream pods that are frequently being cycled.