Update go version to 1.16

This includes a fix for our samesite cookie parsing. The behaviour
changed in 1.16 so that the default value now leaves it empty, so it's
equivalent to not setting it (as per spec)

.github/workflows/ci.yaml

@@ -20,10 +20,10 @@ jobs:
     - name: Check out code
       uses: actions/checkout@v2
 
-    - name: Set up Go 1.15
+    - name: Set up Go 1.16
       uses: actions/setup-go@v2
       with:
-        go-version: 1.15.x
+        go-version: 1.16.x
       id: go
 
     - name: Get dependencies