Relax gem requirement for Rails 7.0 prerelease

[sikachu]

Purpose

This commit updates the upper bound of Rails dependency requirement to < 7.0, as the next version of Rails will be Rails 7.0.

This will allow developers to use and test active_model_serializers against Rails Edge.

Changes

Updates rails_versions in Gemspec to < 7.0.

Caveats

Note that we'll need to bump this upper bound again once Rails 7.0 comes out, as this upper bound will block anything >= 7.0.0.

Additional helpful information

rails/rails@1b455e2

[wasifhossain]

thanks for the PR.