Make multi-version upgrades DB migration job required to pass

What does this MR do and why?

Re-adding !147579 (merged) after revert due to gitlab-org/quality/engineering-productivity/master-broken-incidents#5679 (closed)

• Updates DB migration test for multi-version upgrades to be required to pass
• Documents job description and troubleshooting guidance
• Using DISABLE_DB_MULTI_VERSION_UPGRADE instead of ENABLE_DB_MULTI_VERSION_UPGRADE which caused gitlab-org/quality/engineering-productivity/master-broken-incidents#5679 (closed)

Closes Reinstate "Make multi-version upgrades DB migra... (#452519 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

• Guidance how the test can be run locally https://gitlab.com/gitlab-org/quality/pg-dump-generator#migration-upgrade-testing
• Snowflake metric of the test stability - https://gitlab.com/gitlab-org/quality/pg-dump-generator/-/wikis/home#metrics

Validation

Test has been monitored for 4+ weeks and there were no cases when it failed unexpectedly. See Monitor multi-version upgrades test stability (gitlab-org/quality/quality-engineering/team-tasks#1922 - closed) for more details, specifically gitlab-org/quality/quality-engineering/team-tasks#1922 (comment 1828206842).

✅ Changes in MR validated at https://gitlab.com/gitlab-org/gitlab/-/jobs/6527403883 with !148395 (closed)