Upgrade targetSdkVersion from 31 (Android 12) to 33 (Android 13).

Refs #94.

Looking at Google Play's migration documentation [1] it seems we
might get away with bumping the target sdk level and not changing
anything else \o/

[1] https://developer.android.com/about/versions/13/behavior-changes-13

obelisk/default.nix

@@ -49,7 +49,7 @@ let
       android.platformVersions = [ "29" ];
       android.gradle.minSdkVersion = "27";
       android.gradle.compileSdkVersion = "29";
-      android.gradle.targetSdkVersion = "31";
+      android.gradle.targetSdkVersion = "33";
 
       # Quoth https://developer.android.com/about/versions/12/behavior-changes-12#exported:
       #