CAMEL-20187: Add basic support of virtual threads

[essobedo]

Fixes https://issues.apache.org/jira/browse/CAMEL-20187

Motivation

Java 21 brings virtual threads and we would like to have a preview of it in Camel.

Modification:

• Propose a Java 21 version of DefaultThreadPoolFactory and CamelThreadFactory inside multi-release jars

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[davsclaus]

It would be good to have a bit of doc on this, such as at
https://camel.apache.org/manual/threading-model.html

And we can then try this for 4.3 as a non LTS release to see if we can have it work and build for Gregor.

[orpiske]

This looks cool!!! If we can create a nice pattern for multi-release support, we can potentially use the same approach in a few other areas where this could be useful.