[ruby/irb] Bump version to 1.6.4

(https://github.com/ruby/irb/pull/560)

https://github.com/ruby/irb/commit/01328da9b6

lib/irb/version.rb

@@ -5,7 +5,7 @@
 #
 
 module IRB # :nodoc:
-  VERSION = "1.6.3"
+  VERSION = "1.6.4"
   @RELEASE_VERSION = VERSION
-  @LAST_UPDATE_DATE = "2023-03-06"
+  @LAST_UPDATE_DATE = "2023-04-07"
 end