Add .sdkmanrc for easy Java and Maven version management

[Philzen]

This adds an .sdkmanrc to the project. I've recently found it the most single valuable tools for version management across different projects, as it can ensure each project has exactly the right version of Java and its toolchain when developing and building the project.

Also gets automatically picked up by most modern IDEs, i.e. IntelliJ.

As openapitools requires Java 11, i added its latest BPR here. As the docs do not state which implementation to use, i picked Adoptium, following the recommendation at https://whichjdk.com/

Also added the latest Maven release version. As i understand 3.3.4 is the baseline, kindly advise if that is preferred to ensure nobody accidentally adds features that are incompatible with that version to the POM. Otherwise, my preference would be to go with the latest 3.x version.

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/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  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*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.1.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)