Backfill security_policy_id in compliance_framework_security_policies

What does this MR do and why?

Currently, when a policy is scoped with a compliance framework, we store the policy reference (by policy index in YAML file) and the framework in compliance_framework_security_policies. Now with the security policy read model (&13865) we store the policy in security_policies table.

As a part of Use security policy read model for compliance f... (#481786 - closed), we want to use the read model to link the reference between a security policy and the framework. The issue will have 4 MRs:

1️⃣ Add security_policy_id to compliance_framework_... (!185962 - merged)

2️⃣ Set security_policy_id to compliance_framework_... (!186019 - merged)

3️⃣ Update compliance framework graphql type to rea... (!186080 - closed)

➡️ 4️⃣ Backfill security_policy_id in compliance_frame... (!186097 - merged)

This MR introduces a background migration to backfill security_policy_id to old rows

References

• #481786 (closed)

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