Use generic argument forwarding + remove ruby2_keywords

[chaymaeBZ]

Description

Use generic argument forwarding (...) and remove ruby2_keywords as it is not needed.
This makes it much faster as it does not need to allocate.
This is also a step forward complete removal of ruby2_keywords.

Additional Notes

Can't do the same for the adapter method as it throws a syntax error with ruby 3.0.0.

Code/faraday/lib/faraday/rack_builder.rb:109: syntax error, unexpected (...
  ...dapter(klass = NO_ARGUMENT, ...)
 ...                             ^~~

[olleolleolle]

Thanks, @chaymaeBZ! This streamlines these methods, and takes us into the present!