fix: Play store already has 2.18alpha4, bump version

as part of the release pipeline failure previously, the build version
commit did not land on main, so our version in source on main is stale
vs the version already out in alpha channel, causing all releases to fail
as the version has been used

AnkiDroid/build.gradle

@@ -74,8 +74,8 @@ android {
         //
         // This ensures the correct ordering between the various types of releases (dev < alpha < beta < release) which is
         // needed for upgrades to be offered correctly.
-        versionCode=21800103
-        versionName="2.18alpha3"
+        versionCode=21800104
+        versionName="2.18alpha4"
         minSdk 23 // also in testlib/build.gradle.kts
         // After #13695: change .tests_emulator.yml
         targetSdk 33 // also in [api|testlib]/build.gradle.kts and ../robolectricDownloader.gradle