RUBY-3544 prepare for 2.21.0 (mongodb#2895)

* RUBY-3544 bump version number

* let's bump all the way to 2.21.0

* fix driverbench os spec

Author: jamis

.evergreen/config.yml

@@ -1309,7 +1309,7 @@ buildvariants:
       ruby: "ruby-3.2"
       mongodb-version: "7.0"
       topology: standalone
-    run_on: ubuntu2204
+      os: ubuntu2204
     display_name: DriverBench
     tasks:
       - name: "driver-bench"

.evergreen/config/standard.yml.erb

@@ -48,7 +48,7 @@ buildvariants:
       ruby: <%= latest_ruby %>
       mongodb-version: <%= latest_stable_mdb %>
       topology: standalone
-    run_on: ubuntu2204
+      os: ubuntu2204
     display_name: DriverBench
     tasks:
       - name: "driver-bench"

lib/mongo/version.rb

@@ -16,5 +16,5 @@
 
 module Mongo
   # The current version of the driver.
-  VERSION = '2.20.1'
+  VERSION = '2.21.0'
 end