Update database requirements documentation with more MySQL limitations

What does this MR do?

Updates requirements.md with more MySQL limitations noted in the issue below.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51173#note_99498271

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails => Port https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7390

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/label ~Documentation