Update omniauth-oauth2 requirement from >= 1.5, <= 1.7.1 to >= 1.5, <…

…= 1.7.3

Updates the requirements on [omniauth-oauth2](https://github.com/omniauth/omniauth-oauth2) to permit the latest version.
- [Release notes](https://github.com/omniauth/omniauth-oauth2/releases)
- [Commits](omniauth/omniauth-oauth2@v1.5.0...v1.7.2)

---
updated-dependencies:
- dependency-name: omniauth-oauth2
  dependency-type: direct:production
...

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

omniauth-vkontakte.gemspec

@@ -16,5 +16,5 @@ Gem::Specification.new do |gem|
   gem.require_paths = ['lib']
   gem.version       = OmniAuth::Vkontakte::VERSION
   gem.required_ruby_version = '>= 2.5.0'
-  gem.add_runtime_dependency 'omniauth-oauth2', ['>= 1.5', '<= 1.7.1']
+  gem.add_runtime_dependency 'omniauth-oauth2', '>= 1.5', '<= 1.7.3'
 end