Is your feature request related to a problem? Please describe.

I've found some (older) gems using rubygems_version= or specification_version= in a gemspec file.
However, RubyGems source says not to use them.

DO NOT set this, it is set automatically when the gem is packaged.

https://github.com/rubygems/rubygems/blob/cc6f96aa0c1c99d94363d12884001a2de28da033/lib/rubygems/specification.rb#L530-L534
https://github.com/rubygems/rubygems/blob/cc6f96aa0c1c99d94363d12884001a2de28da033/lib/rubygems/specification.rb#L744-L748

I thought it would be a good idea to handle it the same way as Gemspec/DateAssignment, which was added in #9496.

Describe the solution you'd like

I suggest adding cops like Gemspec/RubyGemsVersionAssignment and Gemspec/SpecificationVersionAssignment that warn the use of rubygems_version= or specification_version= in a gemspec file.

Describe alternatives you've considered

Or merge them with Gemspec/DateAssignment and rename it?

Additional context

For example, Jeweler and the gems created by it (e.g. OS) use these fields.

https://github.com/technicalpickles/jeweler/blob/2ab86309fc2494ba2a4e9c86c514742cd4f681c2/jeweler.gemspec#L193
https://github.com/rdp/os/blob/1245b3a1821c5cf0ff5167069f70b70658752491/os.gemspec#L42