Update webmock requirement from ~> 2.3 to ~> 3.18

Updates the requirements on [webmock](https://github.com/bblimke/webmock) to permit the latest version.
- [Release notes](https://github.com/bblimke/webmock/releases)
- [Changelog](https://github.com/bblimke/webmock/blob/master/CHANGELOG.md)
- [Commits](bblimke/webmock@v2.3.0...v3.18.1)

---
updated-dependencies:
- dependency-name: webmock
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

jekyll-auth.gemspec

@@ -34,5 +34,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rubocop", "~> 0.49", ">= 0.49.0"
   s.add_development_dependency "rubocop-jekyll", "~> 0.11.0"
   s.add_development_dependency "rubocop-performance", "~> 1.0"
-  s.add_development_dependency "webmock", "~> 2.3 "
+  s.add_development_dependency "webmock", "~> 3.18"
 end