Introduce `project_security_statistics` table

What does this MR do and why?

This table will store security statistics like the total number of vulnerabilities for projects.

Database review

Here are the queries introduced by this MR;

Upsert query

INSERT INTO "project_security_statistics" ("project_id")
    VALUES (1)
ON CONFLICT
    DO NOTHING
RETURNING
    "project_id"

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32026/commands/99020

Increase query

UPDATE
    "project_security_statistics"
SET
    vulnerability_count = vulnerability_count + 1
WHERE
    "project_security_statistics"."project_id" = 1

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32026/commands/99021

Decrease query

UPDATE
    "project_security_statistics"
SET
    vulnerability_count = vulnerability_count - 1
WHERE
    "project_security_statistics"."project_id" = 1

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/32026/commands/99022