Update rack-oauth2 to v1.16.0

What does this MR do?

gitlab-foss!30683 (merged) worked around an issue where rack-oauth2 was expecting the client_auth_method to be sent as a Symbol but due to jsonification within Gitlab it was being sent as a String and prevented the value from being configurable.

In rack-oauth2 commit https://github.com/nov/rack-oauth2/commit/cd4be69df80d5e5afc704bf38fc9127ea6ced95b, which was released in v1.14.0, the incoming type is now coerced into a Symbol which obviates the need for a workaround in Gitlab.

In order to use the updated rack-oauth2 gem, a bump to rack was necessary. This has recently been completed in !45183 (merged).

This series bumps rack-oauth2 to v1.16.0 and drops the unnecessary workarounds and tests.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Generally available. I tested via the GDK against a Keycloak instance and had no issues.