gemspec: remove upper Ruby version limit

[bastelfreak]

this limit doesn't make sense since we don't know if the next major version is 4.1 or 5 or 2026-12-31. And dependabot will fail to parse it:

https://github.com/OpenVoxProject/openfact/actions/runs/24180253005/job/70571130540

updater | /home/dependabot/bundler/lib/dependabot/bundler/file_updater/ruby_requirement_setter.rb:95:in 'Kernel#eval': (eval at /home/dependabot/bundler/lib/dependabot/bundler/file_updater/ruby_requirement_setter.rb:95):1: syntax errors found (SyntaxError)
> 1 | '>= 2.5', '< 5.0'
    |         ^ unexpected ',', ignoring it
    |         ^ unexpected ',', expecting end-of-input

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