[Kotlin] Replace deprecated Ktor base64 encoding with Kotlin std library call

[wing328]

based on #23334 with updated samples

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 || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  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)
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Summary by cubic

Replace deprecated io.ktor.util.encodeBase64 with Kotlin stdlib kotlin.io.encoding.Base64 in Kotlin client templates and samples. This removes deprecation warnings and future-proofs generated clients, and fixes #22808.

^{Written for commit 369e6ea. Summary will update on new commits.}

[wing328]

kotlin sample tests passed

[cubic-dev-ai]

No issues found across 12 files