Remove unnecessary rexml dependency from gemspec

[mfittko]

Remove unnecessary rexml dependency from gemspec as it's in ruby's standard gems since 3.0, e.g. https://ruby-doc.org/3.0.6/gems/rexml/REXML.html

We're trying to limit adding unnecessary gem dependencies and since you don't have ruby < 3 any more in your build matrix, I'd suggest to remove it from the dependencies again.

[mfittko]

I know this has been added explicitly in #56 - but this was before 2.7 was removed form the build matrix in #64 @wata727

[wata727]

Thank you for your contribution. But the CI seems to be failing.
https://github.com/wata727/elastic_whenever/actions/runs/7394432951/job/20115799744?pr=67

My understanding is that this dependency is required for Ruby 3.0+, and 2.7 support is not relevant. Since 3.0, rexml is no longer the default gem but a bundled gem, so an explicit dependency is required when using it via Bundler.

[mfittko]

oh now I get it, sorry but we were never having an issue with it because we normally have nokogiri in our projects. let me dig a bit but, I might just close this then. sorry!