As some has mentioned, the problem is that the RN build automatically “upgraded” to androidx.core:core:1.7.0-alpha01, which depends on SDK version 30.

The Fix

The fix is simply to specify android core version via androidXCore in build.gradle

buildscript {
    ext {
        buildToolsVersion = "29.0.3"
        minSdkVersion = 23
        compileSdkVersion = 29
        targetSdkVersion = 29
        ndkVersion = "20.1.5948944"
        kotlin_version = "1.5.0"
        androidXCore = "1.5.0"
    }

How I figured it out

Figuring this out was painful. I grepped for gradle files that would automatically upgrade packages like so

find . -name '*.gradle' -exec grep -H "\.+" {} \;

and found in node_modules/@react-native-community/netinfo/android/build.gradle the following snippet

def androidXCore = getExtOrInitialValue('androidXCore', null)
  if (supportLibVersion && androidXVersion == null && androidXCore == null) {
    implementation "com.android.support:appcompat-v7:$supportLibVersion"
  } else {
    def defaultAndroidXVersion = "1.+"
    if (androidXCore == null) {
      androidXCore = androidXVersion == null ? defaultAndroidXVersion : androidXVersion
    }
    implementation "androidx.core:core:$androidXCore"
  }
}