Remove Unicorn mentions from topology_schema

What does this MR do?

This is an MR split from the bigger Unicorn removal MR.
We want to get rid of everything Unicorn-related (including docs, configs and the gem itself).

It is a separate MR, because it goes into product intelligence area, unlike all other changes.

Feel free to update the text if it doesn't fit.

❗ We opened an issue related to the tracking changes - #331488 (closed) - it is not yet clear for us if some action is required, we'll appreciate the input there A LOT! 🙂

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

Rely on CI and on the product intelligence review here.

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

N/A

Related to #330402 (closed)