[java][okhttp] Default Java client to Java 8 instead of Java 7

Created by: toby-murray-snow-software

My understanding of the version 5 release of OpenAPI generator was that Java 7 support was dropped. Currently the Java client defaults to Java 7, Java 8 can be opted in with the java8 config option. This removes the conditional logic in build.gradle around that config option and defaults to Java 8. This is to support #9151 (closed), up-to-date dependency versions rely on Java 8 and it's easier if that is the default.

To be clear, this changes behavior - previous default version was Java 7, this changes it to Java 8.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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