Dismiss Service for Security Finding

What does this MR do and why?

This MR creates a dismiss service for a security finding, similar to the dismiss service for a vulnerability finding. It needs its own service, however, as the 2 types of findings have some different attributes and behaviors.

The idea here was to mimic https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/vulnerabilities/finding_dismiss_service.rb and https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/vulnerabilities/dismiss_service.rb. Once the dust settles on the feedback removal epic, we can create a base class to simplify the code for these 3 similar classes. My plan is to do minimal work to get this service up for the GraphQL mutation, since it's going to be heavily modified with the feedback removal anyway.

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How to set up and validate locally

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MR acceptance checklist

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Related to #367402 (closed)