Require Jenkins 2.504.3 or newer (#1839)

* Require Jenkins 2.504.3 or newer

https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/#currently-recommended-versions
recommends either Jenkins 2.504.3 or Jenkins 2.516.3 as minimum Jenkins
versions.

We could also increase the minimum Jenkins version to 2.516.3 and have
a longer time before the next increment of Jenkins version, if that
is preferred.

Using Jenkins 2.504.3 as minimum Jenkins version allows us to remove 3
version declarations from the pom file and supersedes pull request:

* https://github.com/jenkinsci/gitlab-plugin/pull/1838

Testing done:

* Confirmed that automated tests pass

* Remove several exclusions that are not needed

Previous updates to the libraries have made those exclusions
unnecessary