Drop CI for <1.9

The plan is to drop support for <1.10, since that's the latest LTS
of julia.

.github/workflows/ci.yml

@@ -22,10 +22,8 @@ jobs:
       fail-fast: false
       matrix:
         version:
-          - '1.6'
-          - '1.7'
-          - '1.8'
           - '1.9'
+          - '1.10'
           - '1' # Leave this line unchanged. '1' will automatically expand to the latest stable 1.x release of Julia.
         os:
           - ubuntu-latest