Use license_files in setup.cfg

Seeing warnings in test runs stating that license_file will be deprecation next month:
```
integration-testing-client-1   |         The license_file parameter is deprecated, use license_files instead.
integration-testing-client-1   | 
integration-testing-client-1   |         By 2023-Oct-30, you need to update your project and remove deprecated calls
integration-testing-client-1   |         or your builds will no longer be supported.
integration-testing-client-1   | 
integration-testing-client-1   |         See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
integration-testing-client-1   |    
```

updating to use `license_files` instead.

setup.cfg

@@ -6,7 +6,7 @@ author = Michael Marchetti
 author_email = [email protected]
 description = Python integration with Posit Connect
 license = GPL-2.0
-license_file = LICENSE.md
+license_files = LICENSE.md
 long_description = file:README.md
 long_description_content_type = text/markdown
 name = rsconnect_python