Add spec for schema.rb

What does this MR do?

It adds a new spec to check:

1. If db/schema.rb defined version equals the latest migration file's timestamp both from db/migration and db/post_migrate directories.
2. If db/schema.rb defined version equals the latest migration's timestamp stored in the database's schema_migrations table.

Are there points in the code the reviewer needs to double check?

I did not use

require Rails.root.join('db', 'schema')

to prevent extra database initializing in the spec.

Why was this MR needed?

It's needed to prevent schema.rb version mismatching.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #21978 (closed)