Update dependabot config to only widen constraints

The docs claim this is the default behaviour for libraries, but doesn't
state anything about how to mark yourself as a library (or how it
detects this). Dependabot immediately created a PR using the "increase"
style when this was added, which isn't useful.

.github/dependabot.yml

@@ -4,3 +4,4 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    versioning-strategy: widen