Add support for Rails 8

[ezekg]

Currently unable to install in a Rails 8 app due to the upper version constraint:

$ bundle add safe_polymorphic
# => Fetching gem metadata from https://rubygems.org/.........
#    Resolving dependencies...
#    Could not find compatible versions
#
#    Because safe_polymorphic < 0.1.3 depends on activerecord >= 5.2, < 7.1
#      and safe_polymorphic >= 0.1.3 depends on activerecord >= 5.2, < 8.0,
#      activerecord >= 5.2, < 8.0 is required.
#    So, because rails >= 8.0.2 depends on activerecord = 8.0.2
#      and Gemfile depends on rails ~> 8.0.2,
#      version solving has failed.