Update the OAS Generator jar version

[nils-work]

Description
The OAS generate script (swagger-gen\oas_generate.sh) specifies an older version of the OpenAPI Generator jar file version used for generating the OAS output in the Standards build process.

A change to use a newer version of the jar for validation was completed in #414 - Update OAS Validator/Generator.

Generate OAS version

- java -jar $OAS_CODEGEN/openapi-generator-cli-5.2.1.jar generate -i $INPUT_SWAGGER -g $OUTPUT_FORMAT -o $SWAGGER_CODEGEN_OUTPUT > $SWAGGER_CODEGEN_OUTPUT/log.txt 2>&1
+ java -jar $OAS_CODEGEN/openapi-generator-cli-6.6.0.jar generate -i $INPUT_SWAGGER -g $OUTPUT_FORMAT -o $SWAGGER_CODEGEN_OUTPUT > $SWAGGER_CODEGEN_OUTPUT/log.txt 2>&1

Key change above is:

- openapi-generator-cli-5.2.1.jar
+ openapi-generator-cli-6.6.0.jar

The newer version provides improved output formatting with clearer nested schema references.