Remove deprecated has_rdoc from gemspec

[larouxn]

This change simply removes the deprecated has_rdoc usage from the gemspec. Due to this deprecated usage end users will see the following message in their console at times. Would be ideal if we can remove this noise.

NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed in Rubygems 4
Gem::Specification#has_rdoc= called from /Users/larouxn/.gem/ruby/ruby-3.3.0/bundler/gems/thrift-b109bdd1a4aa/lib/rb/thrift.gemspec:15.

Looks like it has been deprecated for at least 5 years based on tickets raised about this back in ~2018.

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