Bump master to 9.0 (#5419)

* Bump master to 9.0

* Reset fallbacks to fix tests

* Revert "Reset fallbacks to fix tests"

This reverts commit c1a9451.

Author: neilshweky

lib/mongoid/config/defaults.rb

@@ -50,6 +50,8 @@ def load_defaults(version)
           # All flag defaults currently reflect 8.0 behavior.
         when "8.1"
           # All flag defaults currently reflect 8.1 behavior.
+        when "9.0"
+          # All flag defaults currently reflect 9.0 behavior.
         else
           raise ArgumentError, "Unknown version: #{version}"
         end

lib/mongoid/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Mongoid
-  VERSION = "8.1.0.alpha"
+  VERSION = "9.0.0.alpha"
 end