Update the ndk version used by the testapp builder

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Update the testapp builder's ndk version to 21e, which the Android build of SDK has been using for a while now.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/8180803225

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Type of Change

Place an x the applicable box:

• [ ] Bug fix. Add the issue # below if applicable.
• [ ] New feature. A non-breaking change which adds functionality.
• [x] Other, such as a build process or documentation change.

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