Target `stable` and `oldstable` Go version in CI

[Juneezee]

What type of PR is this?

• maintenance

What this PR does / why we need it:

Align CI configuration with v1-maint (https://github.com/urfave/cli/pull/1887) and v2-maint (https://github.com/urfave/cli/pull/1888)

Which issue(s) this PR fixes:

Closes https://github.com/urfave/cli/issues/1909.

Release Notes

NONE