[BUG] Incorrect example for java resttemplate client

Description

README.md contains incorrect usage guide - it uses class "Configuration" which not generated by this generator (and some others, f.e. webclient).

Looks like usage guide is just copy-paste of "okhttp-gson" guide.

The main problem (from my point of view) that this guide included into generated artifact (I don't think that it's correct to use unexisted classes as exemple for delivered artifact usage)

To be honest I'm not sure what should be adjusted - guide or generator.

openapi-generator version

6.2.0

Steps to reproduce

Just take this example https://github.com/OpenAPITools/openapi-generator/tree/master/samples/client/petstore/java/resttemplate and try to impelemnt it step by step as in README.md

Suggest a fix

Change code in example from
ApiClient defaultClient = Configuration.getDefaultApiClient();
to
ApiClient defaultClient = new ApiClient();

Or adjust generators to produse such class. (Not sure that it's needed)

Start with samples/client/petstore/java/resttemplate/README.md and compare its Configuration-based example with the generated RestTemplate client shown in the sample. Reproduce the README steps, then update the guide or generator only after confirming which source produces the incorrect usage; done means the documented example uses classes present in the generated artifact.