multi: Build and doco updates for Go 1.20.

[davecgh]

This consists of several commits to update the CI and documentation for Go 1.20. It also takes the opportunity to bump to the latest linter and action versions.

In particular:

• build: Update to latest action versions.
  • actions/setup-go@6edd4406fa81c3da01a34fa6f6343087c207a568 #v3.5.0
  • actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c #v3.3.0
• build: Update golangci-lint to v1.51.1.
• build: Test against Go 1.20 and remove Go 1.18 accordingly.
• docs: Update README.md to required Go 1.19/1.20.