Update Go to 1.16.12

Our currently used Go version (1.16.9) is behind a bunch of bugfix releases. We should bump it to 1.16.12 while we are at it. 11.0.3 is about to get built and we might want to include that Go update there, too. To balance the risk of breakage let's land the version bump on master first (despite #40345 (closed)) and test a bit in our nightly series before switching over to 1.17 there.