[ci] upgrade to Go 1.17 everywhere, clean up configs

[imjasonh]

• module integration test now runs with 1.16 and 1.17, and drops an unnecessary matrix value.
• drop unnecessary step names (e.g., "Set up Go 1.17.x", which is redundant with the step config, and will only fall out of date and confuse people), and shell:bash (bash is the default value)

[imjasonh]

gofmt in Go 1.17+ adds //go:build lines corresponding to // +build lines it sees, which is causing the diff. This change is backward compatible for pre-1.17, so we could just gofmt with Go 1.17 to get those lines.

This could be annoying if users are adding new // +build lines and gofmt'ing with Go <1.17, but I don't think that's too likely, and the mitigation is simple -- upgrade your Go version.

wdyt?

[jonjohnsonjr]

I'm ok with deleting the 1.13 thing?

This could be annoying if users

This is for contributors to ko, right? I think that's fine.

[imjasonh]

#446 drops <1.13 support, which is nice because I can't get the boilerplate check to pass with it anyway. 🤷

[codecov-commenter]

Codecov Report

Merging #445 (53344f9) into main (08c6c71) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #445   +/-   ##
=======================================
  Coverage   53.25%   53.25%           
=======================================
  Files          37       37           
  Lines        1842     1842           
=======================================
  Hits          981      981           
  Misses        710      710           
  Partials      151      151           

Impacted Files	Coverage Δ
pkg/build/trimpath_113.go	100.00% <ø> (ø)

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