Include builds for Go 1.18 in CI matrixes

[imjasonh]

I'm keeping Go 1.17.x as the main Go version we use in CI for now, to start slow.

[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[codecov-commenter]

Codecov Report

Merging #1319 (d0cd122) into main (12aeccc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1319   +/-   ##
=======================================
  Coverage   74.19%   74.19%           
=======================================
  Files         113      113           
  Lines        8458     8458           
=======================================
  Hits         6275     6275           
  Misses       1575     1575           
  Partials      608      608           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 12aeccc...d0cd122. Read the comment docs.