docs: correct non-existent bomClassifier element in What's new in Maven 4

[arimu1]

Summary

Fixes apache/maven#12661.

The What's new in Maven 4 page claimed that project BOMs should be generated using a <bomClassifier> element. That element does not exist in the Maven 4.1.0 model or XSD, and there is no example of how to use it.

Investigation

Claim	Reality
<bomClassifier> POM element	Does not exist in maven.mdo / XSD 4.1.0
Import BOM with classifier	Supported for model 4.1.0+ via standard dependency <classifier> (validator skips the "must be empty" rule when is41OrBeyond)
Generating classified BOMs	Done by attaching an additional POM artifact with a classifier (e.g. plugin config such as bom-builder3's bomClassifier parameter) — not a model element

This is a documentation correctness fix only (no model/XSD change).

Changes

• Replace the false <bomClassifier> reference with the real import syntax using <classifier>
• Add a short import example
• Clarify that classified BOMs are attached artifacts, not a POM model element
• Keep the existing guidance about importing only external (non-reactor) BOMs

Testing

• Docs-only change; no code build required
• Reviewed against DefaultModelValidator (classifier allowed for import scope when model ≥ 4.1.0) and confirmed no bomClassifier field in api/maven-api-model

Checklist

• Addresses a single issue without unrelated changes
• Meaningful commit message
• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004