Bump Go version to v1.16
This MR drops the now-unsupported git v2.29.0 build images and starts to build images with Go v1.16 in order to be able to support Gitaly's bumped minimum required Go version of v1.15.
Part of gitaly!3352 (merged)
This MR drops the now-unsupported git v2.29.0 build images and starts to build images with Go v1.16 in order to be able to support Gitaly's bumped minimum required Go version of v1.15.
Part of gitaly!3352 (merged)