jenkins-slave-docker: Ruby 2.4 security maintenance phase has ended

https://www.ruby-lang.org/en/news/2020/03/31/ruby-2-4-10-released/
**

Ruby 2.4 is now under the state of the security maintenance phase, until the end of March of 2020. After that date, maintenance of Ruby 2.4 will be ended. Thus, this release would be the last of Ruby 2.4 series. We recommend you immediately upgrade Ruby to newer versions, such as 2.7 or 2.6 or 2.5.

https://github.com/folio-org/folio-tools/blob/f8044fa6dfb7ddfe9c33ad1c8838ed2873849aaf/jenkins-slave-docker/Dockerfile.focal-java-11

installs Ruby 2.4.2 and 2.4.3.