Fix required_ruby_version in placeholder gemspec

The placeholder dependabot-core.gemspec had required_ruby_version set to
'>= 3', which caused Dependabot's own bundler update jobs to fail.

During update checks, RubyRequirementSetter picks the lowest Ruby version
that satisfies the gemspec constraint. '>= 3' resolved to 3.0.6, which
then got injected into the Gemfile as the Ruby version for dependency
resolution. Since dependabot-common and dependabot-vcpkg require Ruby
>= 3.3.0, every dependency update failed with
'dependency_file_not_resolvable'.

Bump to '>= 3.3.0' to match dependabot-common.gemspec.

Author: JamieMagee

dependabot-core.gemspec

@@ -21,6 +21,5 @@ Gem::Specification.new do |spec|
   }
 
   spec.version = "0.95.1"
-  # Since a placeholder gem, no need to keep `required_ruby_version` up to date with `.ruby-version`
-  spec.required_ruby_version = ">= 3" # rubocop:disable Gemspec/RequiredRubyVersion
+  spec.required_ruby_version = ">= 3.3.0"
 end