[swift5][client] soft deprecate Swift 5 generator

[4brunu]

The Swift 6 generator is stable for a while now, and I think its time that we slowly start to deprecate the swift 5 generator.

I think we should keep it until Xcode stops to support it, but in the meanwhile, we warn people to start migrating to Swift 6.

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)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11) @dydus0x14 (2023/06)

---

Summary by cubic

Deprecated the Swift 5 client generator to guide users to Swift 6. Updated docs and generator metadata to DEPRECATED and added a runtime warning in Swift5ClientCodegen.

^{Written for commit 4090282. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files