Update the ndk version to 21e

[a-maurice]

Description

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

The C++ repo recently changed the ndk version to 21e, so update the Unity repo to use the same.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/2466239666

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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.
• Other, such as a build process or documentation change.

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[DellaBitta]

Is there a CI check on the build for this?

[a-maurice]

Is there a CI check on the build for this?

Yeah, I had run https://github.com/firebase/firebase-unity-sdk/actions/runs/2466239666 from this branch earlier, but before I made the PR, so it didn't get picked up.

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit c596d72
Last updated: Fri Jun 10 18:05 PDT 2022
View integration test log & download artifacts

Failures	Configs
analytics	[BUILD] [ERROR] [2019] [macos] [All 2 Platform(s)] [TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Playmode]
firestore	[TEST] [FLAKINESS] [2019] [macos] [1/5 Platform(s): iOS] [ios_target]
functions	[BUILD] [ERROR] [2019] [macos] [All 2 Platform(s)] [TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Playmode]
messaging	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): iOS] [ios_target]
remote_config	[TEST] [FLAKINESS] [2019] [macos] [1/5 Platform(s): iOS] [ios_target]
storage	[BUILD] [ERROR] [2019] [macos] [All 2 Platform(s)] [TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Playmode]