feat(android) bump minimum API level to 24

Some of our dependencies (most notably WebRTC) have dropped it and we
can no longer claim to support API level 23).

android/build.gradle

 ext {
     buildToolsVersion = "31.0.0"
     compileSdkVersion = 32
-    minSdkVersion    = 23
+    minSdkVersion    = 24
     targetSdkVersion = 32
     supportLibVersion = "28.0.0"