Backport of 'Backfill duo_secret_detection_fp_enabled to false'

What does this MR do and why?

Backport BBM to correct Duo Secret Detection False Positives flow in earlier versions.

duo_secret_detection_fp_enabled was added to project_settings in 18.10 with default: true, null: false. Postgres applied that default to every existing row at migration time, so all pre-existing projects were set to true on upgrade. In 18.11 the default was changed to false via change_column_default, but that only affects new rows. Existing rows kept true, and there was no backfill.

The result: any instance that upgraded through 18.10 has all pre-existing projects set to true. Once the feature is enabled at the group level, those projects are included without anyone making a project-level choice, which defeats the per-project opt-in.

This MR queues a batched background migration that resets the column to false for rows created before the 18.11 default change ran. Rows created afterwards already default to false, so a true value there is a deliberate opt-in and is left untouched. We also leave a row alone when the Secret Detection False Positive Detection foundational flow (secrets_fp_detection/v1) is enabled, since enabling that flow is a deliberate opt-in.

Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/603391

Backports: !241499 (merged)

MR acceptance checklist

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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