Filter non-existing Project IDs while populating `has_vulnerabilities`

What does this MR do?

While testing !46890 (merged) on staging environment, I've found that there are some vulnerability records which are targeting to non-existing projects. Since we are using the vulnerabilities table as the source to collect project IDs, this can cause issues FK constraint failure while trying to insert a project_settings record with a non-existing project ID.

It's possible that the project records can be deleted after we schedule the background jobs therefore we should add an extra check if the projects exist while running the UPSERT query.

This MR fixes this issue.

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

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team