Always populate `finding_data` for Security::Finding test records

What does this MR do and why?

We started always setting the finding_data in application logic a long time ago so there is no point in setting that attribute optionally in specs.

Database review

Old query

SELECT
    1 AS one
FROM
    "security_findings"
    INNER JOIN "security_scans" ON "security_findings"."scan_id" = "security_scans"."id"
WHERE
    "security_scans"."pipeline_id" = 1325549199
    AND (finding_data <> '{}')
LIMIT 1

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29146/commands/90774

New query

SELECT
    1 AS one
FROM
    "security_findings"
    INNER JOIN "security_scans" ON "security_findings"."scan_id" = "security_scans"."id"
WHERE
    "security_scans"."pipeline_id" = 1325549199
LIMIT 1

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/29146/commands/90775