Update omniauth requirement from ~> 1.2 to >= 1.2, < 3.0

Updates the requirements on [omniauth](https://github.com/omniauth/omniauth) to permit the latest version.
- [Release notes](https://github.com/omniauth/omniauth/releases)
- [Commits](omniauth/omniauth@v1.2.0...v2.1.2)

---
updated-dependencies:
- dependency-name: omniauth
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

omniauth-cas.gemspec

@@ -28,5 +28,5 @@ Gem::Specification.new do |gem|
 
   gem.add_dependency 'addressable', '~> 2.8'
   gem.add_dependency 'nokogiri',    '~> 1.12'
-  gem.add_dependency 'omniauth',    '~> 1.9'
+  gem.add_dependency 'omniauth',    '>= 1.9', '< 3.0'
 end