Update python client `openapi-generator-cli` version

[jerabekjiri]

Upgrade openapi-generator-cli to version >=5.0.0.

Currently used python client openapi-generator-cli v4.3.1 contains bug java.time.OffsetDateTime cannot be cast to java.lang.String. This is fixed in version 5.0.0.
Related issues: OpenAPITools/openapi-generator#6694, OpenAPITools/openapi-generator#6433.
PR fix: OpenAPITools/openapi-generator#6458

Taken from pulp/pulpcore#3564 (comment)

Exception in thread "main" java.lang.RuntimeException: Could not process model 'Purge'.Please make sure that your schema is correct!
  	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:496)
  	at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:1005)
  	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:431)
  	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
  	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)
  Caused by: java.lang.ClassCastException: java.time.OffsetDateTime cannot be cast to java.lang.String
  	at org.openapitools.codegen.languages.PythonClientCodegen.toExampleValueRecursive(PythonClientCodegen.java:724)
  	at org.openapitools.codegen.languages.PythonClientCodegen.toExampleValue(PythonClientCodegen.java:703)
  	at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:2846)
  	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:4534)
  	at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:4482)
  	at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:2[378](https://github.com/pulp/pulpcore/actions/runs/4224067517/jobs/7334573002#step:8:379))
  	at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1289)
  	at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:491)
  	... 4 more
  Error: Process completed with exit code 1.

(CI job: https://github.com/pulp/pulpcore/actions/runs/4224067517/jobs/7334573002)

It would be worth upgrading to the latest or at least version >=5.0.0 to fix this, even with possible breaking changes.

[dkliban]

I would like us to upgrade to 6.4.0. I'll start by opening a PR against this repo and use it as a "Required PR" for a pulpcore PR to see what breaks.

[ipanova]

ref pulp/pulpcore#3585