[Shipping new SDK release]: 3.1.0

### new version

3.1.0

### Description

- [x] write release notes documentation in the changelog.MD
- [x] update version in all code examples in main README.md
- [x] update the version in headers in the file /src/Core/ApiClient.php
- [x] local pass phpstan linter
- [x] local pass rector linter
- [x] local pass PHPUnit tests
- [x] pass all integration tests by scope