Synchronize generated gemspec with current bundler template

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See template at https://github.com/rubygems/rubygems/blob/d786a1ccca3922d21572f77b021c51bfaf005819/bundler/lib/bundler/templates/newgem/newgem.gemspec.tt#L34

Additionally, exclude .gem files, since there seem to be several gems built from swagger-codegen that are recursively including older .gem versions, and thus growing in size exponentially

PR checklist

• [x] Read the contribution guidelines.
• [x] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [ ] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

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cc @zlx

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@zlx ping

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Ping again