Build changes to support jdk11+

[agilob]

@wing328 @jimschubert

PR checklist

• [x] Read the contribution guidelines.
• [x] 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.
• [x] Run the following to build the project and update samples:

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

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• [x] File the PR against the correct branch: master (5.3.0), 6.0.x
• [x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[wing328]

cc @OpenAPITools/generator-core-team

[agilob]

bump @OpenAPITools/generator-core-team

[agilob]

@wing328 dont want to rush you with checking this, but could we have these changes in 6.0 please?

[wing328]

@agilob thanks again for the PR 🙏

When you've time, can you please merge the latest master into this branch?

The Gradle publishing workflow has been updated by the Gradle team in the latest master 👍

[agilob]

this should start targetting java17 by now :eyes:

[agilob]

i can try take a look over the weekend

[wing328]

@agilob I've submitted https://github.com/OpenAPITools/openapi-generator/pull/15553 based on this change.

Closing this one for the time being.

Please review https://github.com/OpenAPITools/openapi-generator/pull/15553 when you've time.