chore: switch dependabot strategy to 'lockfile-only'

The "widen" strategy would be ideal, but it isn't supported with uv.
Let's only update the lockfile in that case and deal with constraints
manually.

Author: ahal

.github/dependabot.yml

@@ -4,7 +4,7 @@ updates:
     directory: "/"
     schedule:
       interval: "monthly"
-    versioning-strategy: widen
+    versioning-strategy: lockfile-only
     groups:
       minor-and-patch:
         applies-to: version-updates