fix: Update minSdkVersion to 21 to Fix NDK Compatibility Issue #166

Job	Run time
Build macOS (2.10.5)	4m 23s
Build macOS (2.10.5)	4m 23s
Build iOS (2.10.5)	2m 29s
Build iOS (2.10.5)	2m 29s
Build Android on Ubuntu (2.10.5)	3m 20s
Build Android on Ubuntu (2.10.5)	3m 20s
Build macOS (3.x)	3m 39s
Build macOS (3.x)	3m 39s
Build Android on Windows (2.10.5)	11m 15s
Build Android on Windows (2.10.5)	11m 15s
Build iOS (3.x)	1m 56s
Build iOS (3.x)	1m 56s
Build Android on Ubuntu (3.x)	5m 4s
Build Android on Ubuntu (3.x)	5m 4s
Build Android on Windows (3.x)	10m 6s
Build Android on Windows (3.x)	10m 6s
Build Web on Ubuntu (2.10.5)	1m 0s
Build Web on Ubuntu (2.10.5)	1m 0s
pub.dev scores check	1m 44s
pub.dev scores check	1m 44s
Build Web on Ubuntu (3.x)	1m 17s
Build Web on Ubuntu (3.x)	1m 17s
Build Windows (2.10.5)	3m 20s
Build Windows (2.10.5)	3m 20s
Flutter code style check	31s
Flutter code style check	31s
Build Windows (3.x)	3m 12s
Build Windows (3.x)	3m 12s
android integration test	11m 58s
android integration test	11m 58s
ios integration test	5m 54s
ios integration test	5m 54s
ios integration test with use_frameworks	18m 56s
ios integration test with use_frameworks	6m 57s
macos integration test	3m 7s
macos integration test	3m 7s
pub publish check	21s
pub publish check	21s
windows integration test	6m 6s
windows integration test	6m 6s
Run flutter unit test (2.10.5)	1m 33s
Run flutter unit test (2.10.5)	1m 33s
Run flutter unit test (3.x)	1m 32s
Run flutter unit test (3.x)	1m 32s
Run flutter unit test web (2.10.5)	1m 31s
Run flutter unit test web (2.10.5)	1m 31s
Run flutter unit test web (3.x)	1m 18s
Run flutter unit test web (3.x)	1m 18s
	3h 19m 5s

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