Update to Maven 4.0.0-rc-5 and add version recommendation

Maven 4.0.0-rc-5 includes the fix for the DefaultSourceRoot bug
(apache/maven#10912, backported in #10917) that allows omitting
<directory> elements from <source> declarations when using the
default directory convention src/<module>/<scope>/<lang>.

Changes:
- Update CI workflow to use Maven 4.0.0-rc-5
- Add AsciiDoc include tags for Maven version (single source of truth)
- Add [[recommended-maven4]] admonition in README.adoc
- Add cross-reference to Maven 4 version recommendation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Introduced in the course of support-and-care/maven-support-and-care#137

Author: ascheman

.github/workflows/build.yml

@@ -207,7 +207,9 @@ jobs:
     - name: Set up Maven 4
       uses: stCarolas/setup-maven@v5
       with:
-        maven-version: '4.0.0-rc-4'
+        # tag::maven-4-version[]
+        maven-version: '4.0.0-rc-5'
+        # end::maven-4-version[]
 
     - name: Create .envrc with Java and Maven paths
       shell: bash

README.adoc

@@ -49,7 +49,7 @@ Meanwhile, we extended many examples to work (currently) with Maven 4 to demonst
 ** See <<recommended-jdk>> for the recommended version for full CI (automated testing) compatibility.
 ** For Maven 4 and xref:jigsaw-examples/example_gradle-project/README.adoc[Gradle example]: Java 17 or later is required.
 ** For the xref:jigsaw-examples/example_compile-target-jdk8/README.adoc[cross-compilation example]: Java 8.
-. Install Maven 4 if you want to run the Maven 4 migrated examples (cf. <<maven-4-migration,migration to Maven 4>>).
+. Install Maven 4 if you want to run the Maven 4 migrated examples (cf. <<maven-4-migration,migration to Maven 4>> and <<recommended-maven4>>).
 
 [[recommended-jdk]]
 [NOTE]
@@ -225,6 +225,21 @@ Corresponding JAR files are in `example_.../patchlib`.
 Many examples have been migrated to Maven 4 using the new https://maven.apache.org/plugins-archives/maven-compiler-plugin-LATEST-4.x/modules.html[Module Source Hierarchy] layout.
 The Maven 4 builds are located in `m4/` subdirectories within each example.
 
+[[recommended-maven4]]
+[NOTE]
+.Recommended Maven 4 Version
+====
+The following Maven 4 version is used for CI and automated testing:
+
+[source,yaml,indent=0]
+----
+include::.github/workflows/build.yml[tag=maven-4-version]
+----
+
+Set `M4_HOME` to point to this version.
+For SDKMAN users: `sdk install maven 4...` (use the version shown above).
+====
+
 === Standard Migration Approach and Defaults
 
 For most examples, the Maven 4 migration follows a straightforward pattern.