Add deprecation warning for unsafe_modify method

In #46, we resolved a deadlock in Ruby 3.1 caused by `unsafe_modify`.
We'll deprecate this as it was a fix for one particular issue. Instead,
it's possible to do:

    before do
      @previous_base_url, ENV["BASE_URL"] = ENV["BASE_URL"], "http://localhost:3000"
    end

    after do
      ENV["BASE_URL"] = @previous_base_url
    end

lib/climate_control.rb

@@ -41,6 +41,8 @@ def unsafe_modify(environment_overrides = {}, &block)
     merge(ENV, previous: previous, middle: middle, after: after)
   end
 
+  deprecate :unsafe_modify, :modify, 2024, 11
+
   def env
     ENV
   end