Cap activeadmin at < 4 so AA 4 is an opt-in, not a surprise

ActiveAdmin 4 is a major release and will almost certainly rename or
remove things this gem hooks into (Devise controllers, view overrides,
utility-navigation helpers). With the previous open-ended '>= 3.5'
constraint, users would silently pull AA 4 on their next 'bundle
update' and discover the breakage at boot.

Lock the ceiling to < 4 now. When AA 4 ships, verify the gem against
it, adjust for API changes, and release a new version that bumps the
ceiling to < 5. Standard pattern for ActiveAdmin plugin gems — see
active_admin_sidebar for the same shape.

Author: Fivell

activeadmin-oidc.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |spec|
   ]
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activeadmin",            ">= 3.5"
+  spec.add_dependency "activeadmin",            ">= 3.5", "< 4"
   spec.add_dependency "devise",                 ">= 4.9"
   spec.add_dependency "omniauth",               ">= 2.1"
   spec.add_dependency "omniauth-rails_csrf_protection", ">= 1.0"