Use increase-if-necessary versioning strategy for cargo

It looks like maybe the upstream README is out of date for the cargo
ecosystem, as the code itself would appear to support the strategy
directly and the open issue I found might be able to be closed. Let's
try it out and see.

See:
- https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy
- https://github.com/dependabot/dependabot-core/blob/6a3a59c36b99ea1230fa9b8809c47a7bdd6339ce/cargo/lib/dependabot/cargo/update_checker/requirements_updater.rb#L21
- dependabot/dependabot-core#4009

Author: elasticdog

.github/dependabot.yml

@@ -5,6 +5,7 @@ updates:
     directory: /
     schedule:
       interval: weekly
+    versioning-strategy: increase-if-necessary
 
   - package-ecosystem: github-actions
     directory: /