[swift6][client] Increase minimum supported SDK to accommodate swift concurrency

[4brunu]

[swift6][client] Increase minimum supported SDK to accommodate swift concurrency

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
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  (For Windows users, please run the script in WSL)
  Commit all changed files.
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• 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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@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11) @dydus0x14 (2023/06)

---

Summary by cubic

Raise Swift6 client minimum deployment targets to iOS 13, macOS 10.15, tvOS 13, and watchOS 6 so async/await and Combine are supported without availability guards. Removes redundant @available annotations and updates all generated samples.

• Refactors

  • Updated Package.swift and Podspec templates to the new minimums for non-Vapor clients.
  • Removed @available attributes around async/await methods and Identifiable conformances; regenerated Swift6 samples.

• Migration

  • Consumers must target iOS 13+/macOS 10.15+/tvOS 13+/watchOS 6+ in Xcode, SPM, and Podspec.
  • If you need older OS support, continue using the previous generator version.

^{Written for commit d23b7a9. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 122 files

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed.

[4brunu]

CI failure is not releated to this PR.