Remove the JUnit 4 leftovers

[slachiewicz]

Forward-port of #939 and #940 from wagon-3.x, where this landed first.

The developer guide described the JUnit 3 and 4 shape in four places: wagon-provider-test
dragging in JUnit 3-style PlexusTestCase, WagonTestCase extends PlexusTestCase, getWagon()
calling lookup(Wagon.ROLE, getProtocol()), and the TCK as a JUnit 4 suite wired with
@RunWith(Suite.class). None of that has been true since #936. The guide now also names
junit-platform-suite as a dependency a provider has to add for @Suite; without it the suite
class compiles but selects nothing.

The four junit:junit exclusions no longer exclude anything: removing all four leaves the
reactor resolving the same single junit:junit:4.13.2:test, in wagon-scm, that it resolves
today. That one is why the managed junit:junit version stays, since it is what holds the
maven-scm-test transitive at 4.13.2 rather than 4.12.

The plexus-container-default exclusion in wagon-scm is untouched, and its comment now says
what it actually does: maven-scm 1.11.1 manages that artifact at 1.0-alpha-9, which ships the
same PlexusContainer, DefaultPlexusContainer and PlexusTestCase class names as
org.eclipse.sisu.plexus, and also drags in classworlds 1.1-alpha-2 and junit at runtime
scope.

wagon-tcks/wagon-tck-http/sample-tck-consumer goes: not a reactor module, nothing references
it, pinned at wagon-http-lightweight 1.0-beta-6, and its TestSuite is still
@RunWith(Suite.class), so the one worked example of consuming the TCK shows the wiring the
guide warns against.

Two changes from the wagon-3.x commits do not apply here. TestPrompter and the wagon-ssh
test components.xml exist only on that line, so their comment fixes have nothing to land on.

Verified: dependency:tree over the whole reactor is unchanged, and per-module <testcase>
counts are unchanged against 0962d3e6.

This change was created with AI assistance.