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

[rlnt]

This pull request fixes the usage of the Base64 encoding coming from Ktor with a call to the Kotlin standard library. The old method was marked deprecated with Ktor 3.4.0.

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@karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10) @dennisameling (2026/02)

fixes #22808

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Summary by cubic

Replace deprecated Ktor Base64 encoding (io.ktor.util.encodeBase64) with Kotlin stdlib kotlin.io.encoding.Base64.encode in HttpBasicAuth for Ktor JVM and Multiplatform clients. Removes deprecation warnings and ensures compatibility with Ktor 3.4.0 with no behavior change, fixing #22808.

^{Written for commit 9acac9f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

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[wing328]

please follow step 3 to update the samples so that CI can verify the change

[rlnt]

Problem being I am on Windows, and I have no WSL, nor do I plan to install it just for this PR. I leave it as a draft, and if a maintainer wants to finish it, that would be nice. I only submitted the PR because you asked for it in the linked issue to get things started. Sorry for the inconvenience, but installing a Linux kernel just for a PR is not worth it for me personally 😄

[wing328]

don't worry. we will update the samples later

is it correct to say that you've tested this fix locally to confirm it works for your use cases in the production environment?

[rlnt]

Yes, I used a template override and regenerated the API client to make sure the deprecation message is gone.

[wing328]

merged via #23345 (you as co-author) with updated samples