migrate-from-mysql-using-aws-dms: fix "test" mislabeled as a system database (#23700)

[ti-chi-bot]

This is an automated cherry-pick of #23700

What is changed, added or deleted? (Required)

The doc listed test alongside INFORMATION_SCHEMA, PERFORMANCE_SCHEMA, mysql, and sys under the label "system databases". test is a default user database TiDB/MySQL creates for convenience, not a system (metadata) database like the other four. Reworded the shared label to "default databases", which accurately covers all five entries without changing the practical guidance (still filter out all five when setting up an AWS DMS migration task).

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (LTS)

Do your changes match any of the following descriptions? (Required)

• Delete files in the markdown-pages repository
• Change the directory
• Need modification after machine translation
• Screenshots or GIFs are included in the changes

Summary by CodeRabbit

• Documentation
  • Updated the AWS DMS migration guide to recommend explicitly selecting only the databases and tables intended for migration.
  • Clarified that using the default % wildcard may include MySQL system databases and cause the migration task to fail.

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• tidb-cloud/migrate-from-mysql-using-aws-dms.md

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