multi: Update for Go 1.18.

[davecgh]

This contains several updates to bump to fully support Go 1.18 and use the latest versions of various tooling. In particular:

• Test against Go 1.18.
• Update main module to support Go 1.17 semantics.
• Update to latest action versions:
  • actions/setup-go@f6164bd8c8acb4a71fb2791a8b6c4024ff038dab #v3.0.0
  • actions/checkout@a12a3943b4bdde767164f792f33f40b04645d846 #v3.0.0
  • Uses the pinned hash to help prevent supply chain attacks
• Use recommended golangci-lint installer and update to the latest version 1.45.2.