Remove <optional> from config-file-provider dependency

[strangelookingnerd]

Remove the optional flag from the config-file-provider dependency. To my understanding this dependency is directly referenced in ExecuteDslScripts as well as JenkinsDslScriptLoader and for that reason should not be optional.

Resolves https://github.com/jenkinsci/custom-folder-icon-plugin/issues/280

Testing done

Ran existing tests without any errors.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue