remove CI support for 1.9

Author: NHDaly

.github/workflows/CI.yml

@@ -13,9 +13,8 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - "1.6"  # LTS
+          - "1.6"  # min-supported version
           - "1"    # Latest
-          - "~1.9.0-0"  # To test heap snapshot; remove when v1.9 is latest Julia release.
           - "nightly"
         os:
           - ubuntu-latest