Build: Update mind sdk version to 24

A quick analysis showed that updating to 'minSdkVersion = 24' added
two 'ObsoleteSdkInt' new warnings on the 'aztec' library module.

All these newly created warnings will be dealt with in subsequent
commits. Also, any existing such warnings will be resolved too.

Author: ParaskP7

build.gradle

@@ -63,7 +63,7 @@ subprojects {
 }
 
 ext {
-    minSdkVersion = 21
+    minSdkVersion = 24
     compileSdkVersion = 28
     targetSdkVersion = 31
 }