Enable dependabot and switch GHA references to tags (#168)

### Motivation:

For better control over GitHub workflows, switch over to a tags-based
dependency.

### Modifications:

- Switch to tags
- Enable dependabot to get weekly update checks

### Result:

More streamlined and controlled GHA dependency updates.

Author: czechboy0

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/pull_request.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   soundness:
     name: Soundness
-    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
+    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@0.0.7
     with:
       license_header_check_project_name: "SwiftOpenAPIGenerator"