ci: test against "go.mod", "oldstable", "stable"

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

.github/workflows/test.yml

@@ -25,14 +25,18 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go: ["1.18.x", "oldstable", "stable"]
+        go:
+          - "" # do not specify go-version to fall back on go-version-file (use go.mod); see https://github.com/actions/setup-go/issues/450#issuecomment-3620402646
+          - "oldstable"
+          - "stable"
         platform: [ubuntu-24.04]
     runs-on: ${{ matrix.platform }}
     steps:
     - name: Install Go ${{ matrix.go }}
       uses: actions/setup-go@v6
       with:
         go-version: ${{ matrix.go }}
+        go-version-file: "go.mod" # used when go-version is not specified.
     - name: Setup IPv6
       run: sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=0 net.ipv6.conf.default.disable_ipv6=0 net.ipv6.conf.all.disable_ipv6=0
     - name: Checkout code
@@ -48,14 +52,18 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go: ["1.18.x", "oldstable", "stable"]
+        go:
+          - "" # do not specify go-version to fall back on go-version-file (use go.mod); see https://github.com/actions/setup-go/issues/450#issuecomment-3620402646
+          - "oldstable"
+          - "stable"
         platform: [windows-latest, macos-latest]
     runs-on: ${{ matrix.platform }}
     steps:
       - name: Install Go ${{ matrix.go }}
         uses: actions/setup-go@v6
         with:
           go-version: ${{ matrix.go }}
+          go-version-file: "go.mod" # used when go-version is not specified.
       - name: Checkout code
         uses: actions/checkout@v6
       - name: Build for ${{ matrix.platform }}