Attempt to repair Continuous Integration

[bsiegert]

• use latest Ubuntu instead of ancient 18.04
• use Go 1.20
• remove separate "vet" check. "go test" runs "go vet" as part of the test; enable all vet checks, just to be sure.

Update issue #354

It's a good idea to open an issue first for discussion.

• [x] Tests pass
• [ ] Appropriate changes to README are included in PR

[kele]

Thank you!