Group polars crate updates in dependabot

We depend on `polars` and some of its subcrates explicitly. This leads
to the unfortunate situtation that we have dependabot opening PRs that
are not compiling, independent of whether there are breaking changes we
need to account for. By introducing a [dependabot group](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/customizing-dependency-updates#grouping-dependabot-updates-into-one-pull-request) for `polars` we can at least limit that failure mode.

.github/dependabot.yml

@@ -18,6 +18,14 @@ updates:
     ignore:
       - dependency-name: "*"
         update-types: ["version-update:semver-patch"]
+    groups:
+      # Only update polars as a whole as there are many subcrates that need to
+      # be updated at once. We explicitly depend on some of them, so batch their
+      # updates to not take up dependabot PR slots with dysfunctional PRs
+      polars:
+        patterns:
+          - "polars"
+          - "polars-*"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule: