Update audit event openAPI documentation

What does this MR do and why?

This MR does 2 things:

1. Add date examples to audit event API documentation
2. Add is_array annotation to audit event APIs

The audit event APIs are currently very standard REST resource APIs.

While working on issue 377012, we realized most of the params are simple integer IDs, so they don't gain much from additional documentation.

However, the datetime params seemed like a good candidate for an example:, so those were added.

We also realized we didn't have the is_array annotation on the index routes and added those as well.

Failure codes are not added as they are just standard HTTP codes.

close: https://gitlab.com/gitlab-org/gitlab/-/issues/377012

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.