Update default & secondary Go versions

As discussed below, this MR now switch 1.16.4 to the default and bumps up the 1.15 patch version.

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Old description (forgetting to run gdk update before, likely caused that)

gdk reset-data does not seem to be possible when multiple versions are defined on the same line. I ran into this error just now:

go install -ldflags '-X gitlab.com/gitlab-org/gitaly/internal/version.version=13.12.0-rc1-366-g58261cb08 -X gitlab.com/gitlab-org/gitaly/internal/version.buildtime=20210528.151443' -tags "tracer_static,tracer_static_jaeger,continuous_profiler_stackdriver,static,system_libgit2" gitlab.com/gitlab-org/gitaly/cmd/gitaly-blackbox gitlab.com/gitlab-org/gitaly/cmd/gitaly-wrapper gitlab.com/gitlab-org/gitaly/cmd/gitaly gitlab.com/gitlab-org/gitaly/cmd/gitaly-ssh gitlab.com/gitlab-org/gitaly/cmd/gitaly-backup gitlab.com/gitlab-org/gitaly/cmd/gitaly-debug gitlab.com/gitlab-org/gitaly/cmd/praefect gitlab.com/gitlab-org/gitaly/cmd/gitaly-lfs-smudge gitlab.com/gitlab-org/gitaly/cmd/gitaly-git2go gitlab.com/gitlab-org/gitaly/cmd/gitaly-hooks
go: cannot find GOROOT directory: …/.asdf/installs/golang/1.15.11 1.16.4/go
make[1]: *** [build] Error 2
make: *** […/gitaly/_build/bin/gitaly] Error 2
❌️ ERROR: Failed to reset data.

Since asdf install can parse multiple versions spread over multiple rows, that syntax seems more versatile to use.