Bump test-scope Jetty to the final 9.4 release; Jetty 9 is end of life

Jetty 9, 10 and 11 stopped being published to Maven Central on 1 January 2026 — see [jetty/jetty.project#13918](https://github.com/jetty/jetty.project/issues/13918). Estate-wide context and the shared constraints are in [apache/maven#12768](https://github.com/apache/maven/issues/12768); this issue covers maven-javadoc-plugin only.

## Where we stand

`pom.xml` sets `9.4.55.v20240627` and takes `jetty-server`, `jetty-proxy`, `jetty-servlet` and `jetty-util` at test scope, for the proxy tests.

Test scope, so nothing ships and nothing breaks. The exposure is an unpatched HTTP stack inside our own build, which is a low bar for concern but not nothing.

## What can be done now

**Bump to 9.4.58.v20250814.** That is the final release of the 9.4 line and three patch releases ahead of where we are. Same line, no API change expected, and it is free — there is no reason to sit on 9.4.55 when 9.4.58 is the end of the road and already on Central.

## What cannot be done yet

Jetty 12 requires Java 17. This plugin builds at `javaVersion` 8, and more importantly its CI matrix is pinned to `[ "17", "21", "8", "25" ]`. Test code does not have to obey the shipped baseline — `maven.compiler.testRelease` can be set separately — but it does have to run on every JDK in the matrix, and JDK 8 is in it. So Jetty 12 cannot enter this build at all until that changes.

When it does change, the proxy tests are a good candidate for `org.apache.maven.resolver:maven-resolver-test-http` (published on Central at 2.0.21, already Java 17) rather than a hand-rolled Jetty 12 harness. Worth revisiting then; not actionable today.

*This issue was created with AI assistance.*

Update the jetty.version property in pom.xml from 9.4.55.v20240627 to 9.4.58.v20250814. Start by checking the existing test-scope Jetty dependencies and proxy tests, then run the proxy test suite to confirm the build still passes with the final Jetty 9 release.

Written by the indexing model from the issue text.