artifactory-client Add rexml as an explicit runtime dependency

Description

rexml is a bundled gem since Ruby 3, which means that it needs to be added to the gemspec/Gemfile in order to be required.

The lack of the explicit dependency caused the error LoadError: cannot load such file -- rexml/document in our staging/prod environments, which has been tricky to debug because in dev environments we have test dependencies that depend on rexml and therefore mitigate its absence in the artifactory gemspec.

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

[x] I have read the CONTRIBUTING document.
[x] I have run the pre-merge tests locally and they pass.
[ ] I have updated the documentation accordingly.
[ ] I have added tests to cover my changes.
[ ] If Gemfile.lock has changed, I have used --conservative to do it and included the full output in the Description above.
[x] All new and existing tests passed.
[x] All commits have been signed-off for the Developer Certificate of Origin.