Update GitHub workflows: remove CompatHelper, add Julia dependabot

- Remove CompatHelper workflows (replaced by dependabot)
- Add/update dependabot.yml with Julia package ecosystem
- Julia dependabot support is now stable and preferred over CompatHelper

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Krastanov

.github/dependabot.yml

@@ -1,6 +1,10 @@
-# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
 version: 2
+enable-beta-ecosystems: true
 updates:
+  - package-ecosystem: "julia"
+    directory: "/"
+    schedule:
+      interval: "weekly"
   - package-ecosystem: "github-actions"
     directory: "/" # Location of package manifests
     schedule: