Remove 8.18 from logstash/logstash-plugin CI

[donoghuc]

The 8.18 release track has converged on 8.19. Therefore there is only 8.current. Remove 8.previous from https://github.com/logstash-plugins/.ci/blob/1.x/logstash-versions.yml and ensure that CI is not expecting that key to be in the yaml. Double check both the logstash and logstash plugins CI for anywhere 8.previous may be used or for any instances of depending on 8.18.

[mashhurs]

FYI: I had removed it from (validation matrix) pull-request pipeline in elastic_integration plugin and daily build purely aligns on logstash-versions.yml. That is said, no action need in elastic_integration.