bump jrjackson to 0.4.21 for jruby 10 support (#18862)

this unlocks plugin testing with jruby 10 on main, as bigdecimal dependency in jrjackson must now be explicit

Author: jsvd

versions.yml

@@ -27,6 +27,6 @@ jruby:
 # Note: this file is copied to the root of logstash-core because its gemspec needs it when
 #       bundler evaluates the gemspec via bin/logstash
 # Ensure Jackson version here is kept in sync with version used by jrjackson gem
-jrjackson: 0.4.20
+jrjackson: 0.4.21
 jackson: 2.16.2
 jackson-databind: 2.16.2