libs.versions.toml: add kotlinx-datetime

[dconeybe]

Add "kotlinx.datetime" to libs.versions.toml. Although this PR does not contain any actual uses of the library, a follow-up PR will use it, and I wanted to separate the global change of modifying libs.versions.toml from the code that makes use of it because changing libs.versions.toml has a project-wide blast radius.

See https://github.com/Kotlin/kotlinx-datetime and https://kotlinlang.org/api/kotlinx-datetime/ for more details about the kotlinx.datetime library.

[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

[github-actions]

Vertex AI Mock Responses Check ⚠️

A newer major version of the mock responses for Vertex AI unit tests is available. update_responses.sh should be updated to clone the latest version of the responses: v5.1

[github-actions]

Test Results

 1 022 files  + 1 002   1 022 suites  +1 002   36m 14s ⏱️ + 35m 59s
 5 809 tests + 5 697   5 787 ✅ + 5 675  22 💤 +22  0 ❌ ±0 
11 703 runs  +11 479  11 659 ✅ +11 435  44 💤 +44  0 ❌ ±0 

Results for commit 19b6286. ± Comparison against base commit 22ace25.