go: upgrade to go 1.18

[calebdoxsey]

• https://github.com/pomerium/internal/issues/932

[codecov]

Codecov Report

Merging #31 (47975ed) into main (d755420) will not change coverage.
The diff coverage is n/a.

❗ Current head 47975ed differs from pull request most recent head e394ac4. Consider uploading reports for the commit e394ac4 to get more accurate results

@@          Coverage Diff          @@
##            main     #31   +/-   ##
=====================================
  Coverage   83.6%   83.6%           
=====================================
  Files          1       1           
  Lines         92      92           
=====================================
  Hits          77      77           
  Misses        10      10           
  Partials       5       5