Replace CI version '1.6' with 'lts'

Author: joshday

.github/workflows/CI.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.6'
+          - 'lts'
           - '1' # automatically expands to the latest stable 1.x release of Julia
           - 'nightly'
         os: