Fix vulnerability statistics calculation query

What does this MR do and why?

As we have introduced a change to create vulnerabilities with present_on_default_branch as FALSE, the previous query was counting the vulnerabilities that do not exist on the default branch as well.

With this change, we will start querying the vulnerability_reads table which contains only the vulnerabilities that exist on the default branch.

Related to Project with no vulnerabilities receiving an F ... (#416111 - closed).

Database change

In this MR we are changing an existing query, therefore, I wanted to get database review.

Here is the new query

SELECT
    severity_counts.*,
    (
        CASE WHEN severity_counts.critical > 0 THEN
            4
        WHEN severity_counts.high > 0
            OR severity_counts.unknown > 0 THEN
            3
        WHEN severity_counts.medium > 0 THEN
            2
        WHEN severity_counts.low > 0 THEN
            1
        ELSE
            0
        END) AS letter_grade,
    now() AS created_at,
    now() AS updated_at
FROM (
    SELECT
        vulnerability_reads.project_id AS project_id,
        COUNT(*) AS total,
        COUNT(*) FILTER (WHERE severity = 1) AS info,
        COUNT(*) FILTER (WHERE severity = 2) AS unknown,
        COUNT(*) FILTER (WHERE severity = 4) AS low,
        COUNT(*) FILTER (WHERE severity = 5) AS medium,
        COUNT(*) FILTER (WHERE severity = 6) AS high,
        COUNT(*) FILTER (WHERE severity = 7) AS critical
    FROM
        vulnerability_reads
    WHERE
        vulnerability_reads.project_id IN (278964, 13083)
        AND state IN (1, 4)
    GROUP BY
        vulnerability_reads.project_id) AS severity_counts

Execution plan: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/22842/commands/73707

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.