Bump the stable test to the LTS

Author: ChrisRackauckas

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.2'
+          - '1.6'
           - '1' # automatically expands to the latest stable 1.x release of Julia.
         os:
           - ubuntu-latest