ci: add go1.16

And for windows test against the latest Go version available in chocolatey

.circleci/config.yml

diff --git a/.circleci/config.yml b/.circleci/config.yml
index d9cb79a..1881360 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -8,11 +8,6 @@
     jobs:
       - lint
       - go/test:
-          name: test-golang-1.12
-          executor:
-            name: go/golang
-            tag:  1.12-alpine
-      - go/test:
           name: test-golang-1.13
           executor:
             name: go/golang
@@ -29,17 +24,25 @@
             tag:  1.15-alpine
           codecov-upload: true
       - go/test:
-          name: test-windows-go1.12
+          name: test-golang-1.16
+          executor:
+            name: go/golang
+            tag:  1.16-rc-alpine
+      - go/test:
+          name: test-windows
           executor: windows
           pre-steps:
             - run: |
                 git config --global core.autocrlf false
                 git config --global core.symlinks true
+            - run: |
+                choco upgrade golang
+                go version
 
 executors:
   windows:
     machine:
-      image: windows-server-2019-vs2019:201908-06
+      image: windows-server-2019-vs2019:stable
       resource_class: windows.medium
       shell: bash.exe