Deprecate and remove vulnerability REST APIs
* BE DRI: @subashis
* FE DRI: TBD
## Summary
These are the existing REST APIs for ~"Category:Vulnerability Management":
- https://docs.gitlab.com/ee/api/vulnerabilities.html
- https://docs.gitlab.com/ee/api/vulnerability_findings.html
- https://docs.gitlab.com/ee/api/vulnerability_exports.html
- https://docs.gitlab.com/ee/api/project_vulnerabilities.html
Both of these are in [Alpha](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha) and can be removed outside a major release.
In contrast, the [vulnerability management GraphQL API](https://docs.gitlab.com/ee/api/graphql/reference/index.html#vulnerabilitiescountbyday) is in production and, as per the [GraphQL Vision](https://docs.gitlab.com/ee/api/graphql/#vision), it's the primary means of interacting with ~"Category:Vulnerability Management" objects.
## Goals
1. Ensure ~"Category:Vulnerability Management" REST API features have a GraphQL equivalent.
1. Do not add new REST API endpoints for ~"Category:Vulnerability Management" .
1. Deprecate and remove [`/vulnerabilities/:id`](https://docs.gitlab.com/ee/api/vulnerabilities.html).
1. Deprecate and remove [`/projects/:id/vulnerability_findings`](https://docs.gitlab.com/ee/api/vulnerability_findings.html), `/vulnerabilities/:id/confirm`, `/vulnerabilities/:id/resolve`, `/vulnerabilities/:id/dismiss`.
1. Deprecate and remove [`/security/projects/:id/vulnerability_exports`](https://docs.gitlab.com/ee/api/vulnerability_exports.html#create-a-project-level-vulnerability-export).
1. Deprecate and remove [`/projects/:id/vulnerabilities`](https://docs.gitlab.com/ee/api/project_vulnerabilities.html).
1. Deprecate and remove `vulnerability issue links` related [endpoints](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/api/vulnerability_issue_links.rb). No existing related docs.
1. Allow 6 months between release of equivalent GraphQL functionality and removal of the deprecated REST endpoints.
epic