Remove the Jetty 6 pluginManagement entry and the unbuilt archetype-samples tree

Spun out of the estate-wide Jetty review in [apache/maven#12768](https://github.com/apache/maven/issues/12768), which was triggered by the Jetty 9/10/11 end-of-life announcement in [jetty/jetty.project#13918](https://github.com/jetty/jetty.project/issues/13918). This one is not covered by that announcement — it is older than the versions it covers — but it turned up in the same sweep, and it is cleanup rather than risk.

## What is there

The root `pom.xml` pins `org.mortbay.jetty:maven-jetty-plugin:6.1.26` in `pluginManagement`.

`org.mortbay.jetty` is the pre-Eclipse groupId, and 6.1.26 is the last release ever made under it — November 2010.

## Why it can probably just go

Nothing in the build uses it. The only reference is `archetype-samples/webapp/pom.xml`, in a `jetty` profile with `jetty:run`, and `archetype-samples` is not part of the build:

- it is not listed among the four `` entries (`archetype-models`, `archetype-common`, `maven-archetype-plugin`, `archetype-packaging`)
- it is excluded from RAT (`archetype-samples/**`)
- its own parent is `org.apache.maven.archetypes:archetype-samples:2.0-alpha-5-SNAPSHOT`, a version that was never released, so the tree would not build even if someone tried

To be clear about what this is *not*: the released archetypes live in apache/maven-archetypes, so no generated project inherits any of this. The only theoretical reach is that our root POM is published and its `pluginManagement` is inherited by anything declaring it as a parent — which, outside this repository, is nobody.

## Suggestion

Delete the `pluginManagement` entry and the `archetype-samples` tree together. The samples reference a 15-year-old Jetty and a `tomcat:run` profile from the same era, and they have been unbuildable for long enough that nobody has noticed.

If someone wants the samples kept, they need a live parent and a place in the build; a sample nobody compiles is a sample nobody maintains.

Small and self-contained — reasonable for a first contribution.

*This issue was created with AI assistance.*

Start with the root pom.xml and inspect its pluginManagement entry, then review the archetype-samples tree and its webapp/pom.xml. Remove the obsolete Jetty entry and the unbuilt tree together; done means the four current modules remain unchanged and no build reference to archetype-samples remains.

Written by the indexing model from the issue text.