chore: allow Rails 8.x (< 9.0) in gemspec

Relaxes actionpack/actionview/activerecord upper bounds from < 8.0 to
< 9.0. The jumpstart branch uses no APIs removed in Rails 8.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Charlie Scherer

administrate.gemspec

@@ -13,9 +13,9 @@ Gem::Specification.new do |s|
 
   s.files = Dir["{app,lib,docs}/**/*", "config/locales/**/*", "LICENSE", "Rakefile"]
 
-  s.add_dependency "actionpack", ">= 6.0", "< 8.0"
-  s.add_dependency "actionview", ">= 6.0", "< 8.0"
-  s.add_dependency "activerecord", ">= 6.0", "< 8.0"
+  s.add_dependency "actionpack", ">= 6.0", "< 9.0"
+  s.add_dependency "actionview", ">= 6.0", "< 9.0"
+  s.add_dependency "activerecord", ">= 6.0", "< 9.0"
 
   s.add_dependency "jquery-rails", "~> 4.6.0"
   s.add_dependency "kaminari", "~> 1.2.2"