ci: use dependabot to update julia packages

- applies to root and .buildkite environments
- check every wednesday
- all julia package updates are grouped into a single PR
- remove CompatHelper

Co-authored-by: Claude <noreply@anthropic.com>

Author: haakon-e

.github/dependabot.yml

@@ -1,7 +1,19 @@
 # https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+# Julia support: https://discourse.julialang.org/t/psa-github-dependabot-now-supports-julia/134997
 version: 2
 updates:
   - package-ecosystem: "github-actions"
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"
+  - package-ecosystem: "julia"
+    directories:
+      - "/"
+      - ".buildkite"  # TODO: remove this if/when we move to [workspaces]
+    schedule:
+      interval: "weekly"
+      day: "wednesday"
+    groups: # groups all julia package updates into a single PR
+      all-julia-packages:
+        patterns:
+          - "*"