[BUG][Kotlin-client] Handling default values of parameters

[sjoblomj]

Resolves #10865

Different Kotlin clients currently handle default values of parameters differently; some ignore them, some use the default value. Some also generate invalid Kotlin code, such as @Header("hn1") hn1: java.math.BigDecimal = 71.0 (Double cannot be assigned to BigDecimal).

This PR attempts to make the default values of parameters consistent and without invalid code. It also tidies up the generated comments a little, which unfortunately leads to a lot of changed sample files.

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
  

  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.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jimschubert @dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @wing328

[4brunu]

@sjoblomj Hi, thanks for taking the time to contribute to this project 👍