webrick must be manually installed

`webrick` was removed from the `ruby` `3.0` stdlib, so must be manually
installed.

It's only needed by `pub`, and only as a dev server used in tests, so
added it only for that specific ecosystem as a development dependency.

pub/dependabot-pub.gemspec

@@ -24,6 +24,8 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "dependabot-common", Dependabot::VERSION
 
+  spec.add_development_dependency "webrick", ">= 1.7"
+
   common_gemspec.development_dependencies.each do |dep|
     spec.add_development_dependency dep.name, dep.requirement.to_s
   end