Add logger gem as an explicit dependency

`logger` will be a bundled gem from Ruby 3.5.
https://bugs.ruby-lang.org/issues/20309

So if we use `logger` as the standard library, Bundler shows the
following warning.

```
src/github.com/nov/openid_connect/lib/openid_connect.rb:2: warning: logger was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0
```

This PR adds the `logger` to explicit dependency to fix the warning.

openid_connect.gemspec

@@ -24,6 +24,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency "swd", "~> 2.0"
   s.add_runtime_dependency "webfinger", "~> 2.0"
   s.add_runtime_dependency "rack-oauth2", "~> 2.2"
+  s.add_runtime_dependency "logger"
   s.add_development_dependency "rake"
   s.add_development_dependency "rspec"
   s.add_development_dependency "rspec-its"