Revert "Changed: Bump compileSdkVersion to 31"

This reverts commit 296ee60

We do not need to bump to compileSdkVersion 31 currently, since I have decided not to bump `androidx.window` to `1.0.0-alpha10` or higher currently, since it has changed APIs and ViewUtils will break.

https://developer.android.com/jetpack/androidx/releases/window

Moreover, bumping compileSdkVersion to 31 requires openjdk 11 in build environment, which will break Jitpack library build and possibly still F-Droid as well, unless changes are made.

https://gitlab.com/fdroid/fdroiddata/-/issues/2441

jitpack/jitpack.io#4474

https://jitpack.io/docs/BUILDING/#java-version

Also in android studio stub files are loaded when opening class sources since android 12 sources aren't available.

gradle.properties

@@ -18,7 +18,7 @@ android.useAndroidX=true
 minSdkVersion=24
 targetSdkVersion=28
 ndkVersion=22.1.7171670
-compileSdkVersion=31
+compileSdkVersion=30
 
 markwonVersion=4.6.2