Fix Go the versions used by our workflow tests

[brackendawson]

Current tests

We currently run full tests ("build") against:

• stable (1.22 today)
• oldstable (1.21 today)

Then partiual tests ("test") without formatting (because gofmt changed in 1.19) against:

• 1.17
• 1.18
• 1.19
• 1.20
• 1.21

It would be nice to not have to keep updating this file with new Go versions, because human processes fail.

Proposed

I think we can use this set long term:

• build: stable (because it is the latest supported Go release).
• build: oldstable (because it is the other supported Go release).
• build: 1.21 (because it is the oldest Go release which will fail to compile if a dependency has a go.mod go directive newer than the toolchain).
• test: module (currently 1.17 because it is the version we declare support for in our go.mod, we then don't have to have the comment in go.mod).