Fix Ruby 3.3 deprecation warning

Ruby 3.3 [moved][] the `observer` library into a bundled gem, which
means we must declare it as an explicit depedency going forward.

[moved]: https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/

Gemfile.lock

@@ -3,6 +3,7 @@ PATH
   specs:
     factory_bot (6.4.2)
       activesupport (>= 5.0.0)
+      observer
 
 GEM
   remote: https://rubygems.org/
@@ -73,6 +74,7 @@ GEM
     middleware (0.1.0)
     minitest (5.14.3)
     multi_test (0.1.2)
+    observer (0.1.2)
     parallel (1.22.1)
     parser (3.1.2.1)
       ast (~> 2.4.1)

factory_bot.gemspec

@@ -27,6 +27,7 @@ Gem::Specification.new do |s|
   }
 
   s.add_dependency("activesupport", ">= 5.0.0")
+  s.add_dependency("observer") if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.3')
 
   s.add_development_dependency("activerecord")
   s.add_development_dependency("appraisal")