feat: test on stable and oldstable Go versions

Author: twpayne

.github/workflows/main.yml

@@ -10,10 +10,18 @@ on:
     - v*
 jobs:
   test:
+    strategy:
+      fail-fast: false
+      matrix:
+        go-version:
+        - stable
+        - oldstable
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11
     - uses: actions/setup-go@0c52d547c9bc32b1aa3301fd7a9cb496313a4491
+      with:
+        go-version: ${{ matrix.go-version }}
     - name: build
       run: go build ./...
     - name: test