Add index to support vulnerability_occurrences.uuid type migration

What does this MR do and why?

Add index to support vulnerability_occurrences.uuid type migration. We need this index, otherwise the batch selection when scheduling the batched background migration might fail

Related to #335708 (closed)

Database review

Before

SELECT min(id)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65056

SELECT max(id)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65065

After

SELECT min(id)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65053

SELECT max(id)

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19904/commands/65054

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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