Add Docker-DB 8.18.1

[tkilias]

Summary

A new Docker-DB version which we need to support have been released.

Task(s)

• [x] Add config for Docker-DB 8.18.1
• [x] Change default image version to 8.18.1
• [x] Update LATEST_DB_VERSION in test_environment_options.py
• [x] Update currently supported version(s) in user_guide.rst
• [x] Update changelog

[ckunki]

Please see Review

[ckunki]

Thanks to @tkilias for comments. Marked as approved.