android: Update targetSdkVersion to 34, i.e., Android 14

This change will be required in order to upload new releases to the
Play Store, effective 2024-10-31, which includes the extension we
requested.

This change causes Android 14 and later to apply to our app a number
of behavior changes introduced in that version, documented here:
  https://developer.android.com/about/versions/14/behavior-changes-14

Fixes: zulip#5877

android/build.gradle

@@ -13,7 +13,7 @@ buildscript {
         // but some testing is required and code changes are often required.
         // See upstream for background and more discussion:
         //   https://medium.com/androiddevelopers/picking-your-compilesdkversion-minsdkversion-targetsdkversion-a098a0341ebd
-        targetSdkVersion = 33
+        targetSdkVersion = 34
 
         // Should be the latest SDK version available.  See upstream recommendation:
         //   https://medium.com/androiddevelopers/picking-your-compilesdkversion-minsdkversion-targetsdkversion-a098a0341ebd