fix(dependabot): remove invalid runs-on field (schema violation)

The field is not part of the supported dependabot.yml schema. Pre-emptive
fix to unblock future PRs that touch dependabot.yml — validation only
triggers then.

Author: JCBauza

.github/dependabot.yml

@@ -10,7 +10,6 @@ updates:
         patterns: ["*"]
       all-dependencies:
         patterns: ["*"]
-    runs-on: "ubuntu-latest"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
@@ -21,4 +20,3 @@ updates:
         patterns: ["*"]
       all-actions:
         patterns: ["*"]
-    runs-on: "ubuntu-latest"