Add migration user

What does this MR do?

This MR introduces a new internal bot user, the migration bot. The intention is to use this user in migrations or operations where a generic user should be used. For example in the snippets backfilling migration.

Refs #216120 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [ ] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content