Fix locations for vulnerabilities created via api

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR fixes #347145 (closed).

Vulnerability creation APIs were setting finding.raw_metadata to a Hash. However, raw_metadata is a text field rather than jsonb, which means that raw_metadata needs to be converted to JSON. This MR does the following:

1. Begin making using of the location field in vulnerability creation APIs. This field is jsonb rather than text and will eventually replace raw_metadata.
2. Fix how the vulnerability creation APIs populate raw_metadata by making them pass JSON rather than a Hash.
3. Create a background migration which fixes the existing records in the database.

Query Plans

Purpose	Query	Plan Link
Queue batches of IDs	SELECT "vulnerability_occurrences"."id" FROM "vulnerability_occurrences" WHERE "vulnerability_occurrences"."report_type" IN (7, 99) AND "vulnerability_occurrences"."id" >= 1 ORDER BY "vulnerability_occurrences"."id" ASC LIMIT 500	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7854/commands/28054
Get min and max ID	SELECT MIN("vulnerability_occurrences"."id"), MAX("vulnerability_occurrences"."id") FROM "vulnerability_occurrences" WHERE "vulnerability_occurrences"."report_type" IN (7, 99) AND "vulnerability_occurrences"."id" >= 20000000 AND "vulnerability_occurrences"."id" < 20000500	https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/7854/commands/28056

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