Update Kotlin version in UDP Exporter Sample Test App

[jj22ee]

Issue #, if available:

• Fix failing workflow udp-exporter-e2e-test, where the sample app is unable to be built
  • https://github.com/aws-observability/aws-otel-java-instrumentation/actions/runs/17587060722/job/49957841960

FAILURE: Build failed with an exception.

* What went wrong:
Could not determine the dependencies of task ':compileKotlin'.
> Unknown Kotlin JVM target: 21

Description of changes:

• Update Kotlin version in UDP Exporter Sample Test App. It is also now aligned with the Kotlin version used in other build.gradle.kts files in this repo.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.07%. Comparing base (09e6487) to head (de209cb).
⚠️ Report is 484 commits behind head on main.
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Additional details and impacted files

@@              Coverage Diff              @@
##               main    #1184       +/-   ##
=============================================
- Coverage     85.71%   67.07%   -18.64%     
- Complexity       19      525      +506     
=============================================
  Files             3       54       +51     
  Lines            49     2694     +2645     
  Branches          5      376      +371     
=============================================
+ Hits             42     1807     +1765     
- Misses            3      750      +747     
- Partials          4      137      +133     

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