Adjust minimum Ruby version in the gemspec to match documentation, CI, and Changelog

[kpumuk]

We have documented the minimum Ruby version as 2.7 everywhere, CI does not test below it. Thrift 0.23.0 will be the last release that can be installed on older Ruby versions, starting with 0.24.0 user will be forced to upgrade Ruby to at least 2.7

Not marking this change as breaking since we already announced that Ruby 2.7 is effectively required.

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• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
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• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.