CI: Turn off mixed version tests running Khepri

Khepri is experimental and not guaranteed to be upgradable. We should
re-enable mixed version testing as Khepri stabilizes but for now we
should turn it off.

This saves some CI time and avoids some failures that will be introduced
when we upgrade Khepri to v0.14.0.

(cherry picked from commit e08310a)

Author: the-mikedavis

.github/workflows/test-plugin-mixed.yaml

@@ -29,7 +29,10 @@ jobs:
         - 26
         metadata_store:
           - mnesia
-          - khepri
+          # Khepri is currently skipped because Khepri is an unstable feature: we don't guarantee upgrability.
+          # Mixed-version tests currently fail with Khepri because of a new machine version introduced in
+          # Khepri v0.14.0.
+          # - khepri
         include:
         - erlang_version: 26
           elixir_version: 1.15