build(ci): test matrix include go1.12 up to go.18

This tests back from the version specified in go.mod to current stable.
This should enable detection of any breaking version changes as part of
the dependencies update PR.

subtask forked from lestrrat-go#28.

.github/workflows/ci.yml

@@ -6,21 +6,21 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go: [ '1.15', '1.14' ]
+        go: [ '1.18', '1.17', '1.16', '1.15', '1.14', '1.13', '1.12' ]
     name: Go ${{ matrix.go }} test
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
       - name: Install Go stable version
-        uses: actions/setup-go@v2
+        uses: actions/setup-go@v3
         with:
           go-version: ${{ matrix.go }}
       - name: Install GolangCI-Lint
         run: curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin v1.25.0
       - name: Test with coverage
         run: make cover
       - name: Upload code coverage to codecov
-        if: matrix.go == '1.15'
+        if: matrix.go == '1.18'
         uses: codecov/codecov-action@v1
         with:
           file: ./coverage.out