Revert "Release: Thunderbird Beta 13.0b1", as release is unnecessary

This reverts commit 08bccc7.

Author: dandarnell

app-metadata/net.thunderbird.android.beta/en-US/changelogs/28.txt

@@ -17,7 +17,7 @@ android {
         applicationId = "net.thunderbird.android"
         testApplicationId = "net.thunderbird.android.tests"
 
-        versionCode = 28
+        versionCode = 27
         versionName = "13.0"
 
         buildConfigField("String", "CLIENT_INFO_APP_NAME", "\"Thunderbird for Android\"")