[FR]: Replace spotless with Ktlint/Ktfmt

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the problem

The spotless plugin is creating some build performance issues.

### Describe the solution

We should move to ktlint or ktfmt.

Block just shared a timely post: https://engineering.block.xyz/blog/adopting-ktfmt-and-detekt

### Additional context

Note that the alternatives don't support everything spotless does, like checking the copyright headers.

Any feedback is welcome

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by reviewing the current Spotless configuration and comparing ktlint with ktfmt, especially their support for copyright headers. The issue does not name files or tests; done would require selecting an alternative, preserving required formatting checks, and addressing the reported build-performance concern.

Written by the indexing model from the issue text.