Bump activeadmin floor to >= 3.5

The true compatibility floor is ActiveAdmin 3.5, not 3.0: 3.4 and
earlier cap devise at < 5, which conflicts with the devise >= 4.9
dependency declared here (bundler resolves it to devise 5.x on a
clean install). Users on AA 3.4 hit "You provided devise 5.0.3 but
we need: >= 4.0, < 5" at boot.

Matching the gemspec to what CI actually exercises.

Author: Fivell

activeadmin-oidc.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |spec|
   ]
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activeadmin",            ">= 3.0"
+  spec.add_dependency "activeadmin",            ">= 3.5"
   spec.add_dependency "devise",                 ">= 4.9"
   spec.add_dependency "omniauth",               ">= 2.1"
   spec.add_dependency "omniauth-rails_csrf_protection", ">= 1.0"