[ruby/irb] Bump version to 1.8.1

(https://github.com/ruby/irb/pull/706)

https://github.com/ruby/irb/commit/c7c838a4bf

lib/irb/version.rb

@@ -5,7 +5,7 @@
 #
 
 module IRB # :nodoc:
-  VERSION = "1.8.0"
+  VERSION = "1.8.1"
   @RELEASE_VERSION = VERSION
-  @LAST_UPDATE_DATE = "2023-08-30"
+  @LAST_UPDATE_DATE = "2023-09-04"
 end