d/rules: enable build against ruby3.0

Lucas Kanashirorequested to merge
kanashiro/libguestfs:add-ruby3.0 into debian/master

We started the transition to ruby 3.0 [1] and libguestfs is not correctly building the ruby binding with the new ruby. I noticed that RUBY_VERSIONS in d/rules was not finding the new version.

[1] https://release.debian.org/transitions/html/ruby3.0-add.html

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