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 <noreply@anthropic.com>

Author: Krastanov

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+enable-beta-ecosystems: true
+updates:
+  - package-ecosystem: "julia"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"