ci/gha: update

1. Update Go versions used in tests to current stable releases
   (plus 1.13 which is still used by Moby).

2. Explicitly specify ubuntu version used.

3. Bump actions/setup-go to v2.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -12,12 +12,12 @@ jobs:
     name: Build
     strategy:
       matrix:
-        go-versions: [1.12.x, 1.13.x, 1.14.x]
-        platform: [ubuntu-latest]
+        go-versions: [1.13.x, 1.15.x, 1.16.x]
+        platform: [ubuntu-20.04]
     runs-on: ${{ matrix.platform }}
     steps:
     - name: Install Go
-      uses: actions/setup-go@v1
+      uses: actions/setup-go@v2
       with:
         go-version: ${{ matrix.go-version }}