.github: use proper name for Mac OS 11 (10.16 was unofficial; we could also use 10.15)

.github/workflows/build-macos.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   build-macos:
     name: build-macos
-    runs-on: macos-10.16
+    runs-on: macos-11
     steps:
       - name: Install Go
         uses: actions/setup-go@v2