Extend go-version to accept go.mod files

[myaaaaaaaaa]

Description:
Right now, matrix testing ['go.mod', 'stable'] is somewhat awkward since it involves deliberately setting go-version to an empty string:

jobs:
  tests:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        go-version: ['', 'stable']

    steps:
    - name: Setup Go
      uses: actions/setup-go@v5
      with:
        go-version: ${{ matrix.go-version }}
        go-version-file: go.mod

This PR makes such a matrix test more straightforward:

jobs:
  tests:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        go-version: ['go.mod', 'stable']

    steps:
    - name: Setup Go
      uses: actions/setup-go@v5
      with:
        go-version: ${{ matrix.go-version }}

Related issue:
Fixes #450

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.