Upgrade NDK version for CI build error

[jeesang7]

Upgrade NDK version for build error in the following log in CI.

...
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:stripReleaseDebugSymbols'.
> No version of NDK matched the requested version 21.4.7075529. Versions available locally: 23.2.8568313, 24.0.8215888, 25.2.9519[65](https://github.com/flutter-webrtc/flutter-webrtc-demo/actions/runs/6173310379/job/16755352823#step:7:66)3, 25.2.9519653

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org/

BUILD FAILED in 3m 54s
Running Gradle task 'assembleRelease'...                          235.0s
Gradle task assembleRelease failed with exit code 1
Error: Process completed with exit code 1.