Add an alias for the previous stable Go release

Description: I’d like setup-go to support stable and oldstable as Go version identifiers. stable would resolve to the latest patch release of the current stable minor release (so 1.18.x today), and oldstable would resolve to the latest patch release of the previous minor release (so 1.17.x today). These aliases make it easy for projects to offer the same support policy as the Go language.

Justification: With semver ranges, it’s easy to express “use the latest Go release”: ^1.18.1 works. However, this is an awkward approach for Go projects:

• Go doesn’t use this style of semantic versioning for dependency management, so the syntax is unfamiliar to much of the target audience.
• Many projects mirror Go’s own support policy and want to test on both stable and oldstable. Expressing this today requires changing the workflow definition every 6 months.
• Travis CI’s gimme has supported this for years, starting with https://github.com/travis-ci/gimme/pull/170 from @abhinav.

I’m not attached to the stable and oldstable names in particular (I think Travis CI wanted oldstable because it’s similar to a Debian convention) - happy to use whatever you’d like.

Are you willing to submit a PR? Happy to.

[IvanZosimov]

Thank you for your request, @akshayjshah ! We will investigate the possibility of implementing this feature and get back to you with some updates 👍

[akshayjshah]

Works as expected - thanks for implementing and releasing this! 😻