.github/workflows: add Go 1.17 to test matrix

Drop 1.15 which is no longer supported upstream.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.15, 1.16]
+        go-version: [1.16, 1.17]
         platform: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.platform }}
 
@@ -26,11 +26,11 @@ jobs:
       uses: actions/checkout@v2
 
     - name: Check formatting
-      if: matrix.go-version == '1.16' && matrix.platform == 'ubuntu-latest'
+      if: matrix.go-version == '1.17' && matrix.platform == 'ubuntu-latest'
       run: diff -u <(echo -n) <(go fmt $(go list ./...))
 
     - name: Check Go modules
-      if: matrix.go-version == '1.16'
+      if: matrix.go-version == '1.17'
       run: |
         go mod tidy
         git diff --exit-code