Find or initialize Scanners using project_id

What does this MR do and why?

Add project_id as argument to find_or_initialize_by so the scanners are created in the correct project if they don't exist within that project already.

This prevents the following scenario:

1. No scanner called gitlab-manual-vulnerability-report exists in any project.
2. Project A creates a vulnerability manually.
3. gitlab-manual-vulnerability-report scanner gets created within project A.
4. Project B creates a vulnerability manually.
5. Project B users don't see any scanner attached to the finding because it's actually attached to the scanner created in step 3 and they don't have access to that scanner.

Fixing this 100% will require a data migration which will be performed later on.

Related to #355802 (closed)

How to set up and validate locally

1. Create a new project (assuming project-a)
2. Go to http://127.0.0.1:3000/root/project-a/-/security/vulnerabilities/new
3. Create a new Vulnerability
4. Create another project (assuming project-b)
5. Go to http://127.0.0.1:3000/root/project-b/-/security/vulnerabilities/new
6. Create a new Vulnerability
7. Go to http://127.0.0.1:3000/-/graphql-explorer
8. Issue the following query

query {
  project(fullPath:"root/project-a") {
    vulnerabilityScanners {
      nodes {
        id
        name
        reportType
      }
    }
    vulnerabilities {
      nodes {
        id
        scanner {
          externalId
          id
          name
          reportType
          vendor
        }
      }
    }
  }
}

1. Repeat for project-b
2. Make sure both queries have a non-empty vulnerabilityScanners and vulnerabilities.scanner.id DO NOT match

MR acceptance checklist

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