[BUG][KOTLIN] Refactor type mappings to map types into simple class names

[CaptainAye]

This PR is a follow up pr for #19981 and bug #19942.
During the initial fix preparation, I discovered that even though DefaultCodegen and other language codegens (i.e. java) expect the mapped type in typeMapping to be a simple class name, kotlin codegens are putting canonical names in there.
Then, the {{dataType}} model param is eventually resolved to the canionical name, whereas it is expected to be a simple class name.
This situation can lead to the compilation errors from the generated code in the worst case scenario.
This PR aims to fix this issue by changing typeMapping to simple class names and introducing importMapping whenever the canionical name is required in the imports.

For convenience, the PR contains 2 commits. The first one contains all the code changes, whereas the second one is mostly the regenerated samples.

Technical Kotlin Committee:
@dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l

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.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[4brunu]

This was attempted in the past #4794
@wing328 and @jimschubert I would like to have your input on this one please.