chore: switch from custom update workflow to Dependabot groups

Replace the custom update-packages.yml workflow with Dependabot's
native grouped updates. This gives us batched PRs that trigger CI
natively, security vulnerability alerts, and less custom code to
maintain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JerrettDavis

.github/dependabot.yml

@@ -18,6 +18,9 @@ updates:
       prefix: "deps"
       include: "scope"
     groups:
+      all-nuget:
+        patterns:
+          - "*"
       microsoft:
         patterns:
           - "Microsoft.*"
@@ -57,3 +60,7 @@ updates:
     commit-message:
       prefix: "ci"
       include: "scope"
+    groups:
+      all-actions:
+        patterns:
+          - "*"