fix: CI Unity 2020 Android API level

[bitsandfoxes]

Follow up on #2231 - This slipped through when updating the Builder.cs

The default API level in Unity 2020 is 19, leading to the following error

  10:41:30.915 | /sentry-unity/samples/IntegrationTest/Temp/gradleOut/launcher/src/main/AndroidManifest.xml Error:
  10:41:30.915 | 	uses-sdk:minSdkVersion 19 cannot be smaller than version 21 declared in library [:sentry-android-ndk-release:] /home/gh/.gradle/caches/transforms-2/files-2.1/8d26493c05c6c8bf9a67a4b46bd75e75/sentry-android-ndk-release/AndroidManifest.xml as the library might be using APIs not available in 19
  10:41:30.916 | 	Suggestion: use a compatible library with a minSdk of at most 19,
  10:41:30.916 | 		or increase this project's minSdk version to at least 21,
  10:41:30.916 | 		or use tools:overrideLibrary="io.sentry.android.ndk" to force usage (may lead to runtime failures)

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