Beginning with AndroidX.Core 1.7.0, the Google AndroidX Java packages require API-31 (Android 12) in order to compile. Attempting to compile apps using them with previous versions of Android result in the following error:

error APT2260: resource android:attr/lStar not found.
error APT2062: failed linking references.

This can also be seen in the META-INF\com\android\build\gradle\aar-metadata.properties file in core-1.7.0.aar (source):

aarFormatVersion=1.0
aarMetadataVersion=1.0
minCompileSdk=31

AndroidX.Core is a "core" part of AndroidX and thus nearly every AndroidX package depends on it. As such, we have to move the entire AndroidX package ecosystem from MonoAndroid9.0/net6.0-android31.0 to MonoAndroid12.0/net6.0-android31.0. This means if you are targeting MonoAndroid9.0/10.0/11.0, you will not be able to use the latest packages. However the existing packages based on AndroidX.Core 1.6.x will continue to work.

Note this only affects the Android version you are "targeting". Applications targeting MonoAndroid12.0 will continue to run on devices running previous Android versions.