Stop limiting json-schema version

This was causing conflicts in applications using `json-schema` 5.x.

The only breaking change in 5.0 was dropping support for old Ruby
versions, which we don't support either.

mcp.gemspec

@@ -28,5 +28,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_dependency("json_rpc_handler", "~> 0.1")
-  spec.add_dependency("json-schema", "~> 4.1")
+  spec.add_dependency("json-schema", ">= 4.1")
 end