test: using stable/oldstable aliases

If stable is provided, action will get the latest stable version from
the go-versions repository manifest.

If oldstable is provided, when current release is 1.19.x, action will
resolve version as 1.18.x, where x is the latest patch release.

We also want to maintain 1.20 version, so we test it.

[#186180915](https://www.pivotaltracker.com/story/show/186180915)

.github/workflows/run-tests.yml

@@ -15,7 +15,7 @@ jobs:
  runs-on: ubuntu-latest
  strategy:
  matrix:
- version: [ '1.20', '1.21' ]
+ version: [ 'stable', 'oldstable', '1.20' ]
  name: Go ${{ matrix.version }}
  outputs:
  pr_number: ${{ github.event.number }}