Readme and 3.0.0 migration guide reference non-existing classes

### Bug Description

The [migration guide](https://github.com/CycloneDX/cyclonedx-gradle-plugin/releases/tag/cyclonedx-gradle-plugin-3.0.0) and the README both reference a package called `org.cyclonedx.model.schema` which does not exist. The enum `SchemaVersion` also does not exist. What _does_ exist is an enum called `Version` but it's in `org.cyclonedx`.

https://github.com/CycloneDX/cyclonedx-core-java/blob/master/src/main/java/org/cyclonedx/Version.java

I believe both the README and the migration guide need to be updated.

### Self-contained Reproducer Project

n/a

### Expected Behavior

n/a

### Gradle build scan URL (optional)

_No response_

### OS

Liniux

### Gradle version

9.2

### CycloneDX Plugin version

3.1.0

### Additional Context

I can provide a PR for the README but I can't change the migration guide. I'll hold off until you confirm what you'd like to see. I'm unsure where these wrong package names are coming from and maybe I'm just missing something?!

### Contribution

- [x] I am willing to provide a fix
- [x] I will wait until somebody else fixes it