Record the Maven 3.9.0 baseline in the README and site

Both still advertised 3.6.3 with 3.9.0 needed only to release, which 27
changed. The README also gains the version.<artifactId> convention, since
that is how a child overrides a managed plugin version now.

Author: slachiewicz

README.md

@@ -38,7 +38,7 @@ that element to point at its own `gh-pages` branch.
 
 The parent POM covers the following areas:
 
-- **Baselines**: Java 8 through the `javaVersion` property, Maven 3.6.3 as the minimum, and UTF-8 sources.
+- **Baselines**: Java 8 through the `javaVersion` property, Maven 3.9.0 as the minimum, and UTF-8 sources.
 - **Enforcer**: the Maven and JDK minimums, and a rule from `extra-enforcer-rules` that rejects a
   dependency whose bytecode is newer than your compiler target.
 - **Formatting**: Spotless with palantir-java-format, sorted POM files, and flexmark for Markdown.
@@ -48,15 +48,17 @@ The parent POM covers the following areas:
 - **Reporting**: the `reporting` profile adds Javadoc, JXR, surefire, PMD/CPD, and taglist.
 - **Releasing**: the `plexus-release` profile adds GPG signing, sources, and a source-release assembly,
   and enables [Njord](https://maveniverse.eu/docs/njord/) to publish to Maven Central.
+- **Version properties**: every managed plugin version has a `version.<artifactId>` property, so a child
+  can pin a different version without redeclaring the plugin.
 
 For an explanation of each area, see the [project site](https://codehaus-plexus.github.io/plexus-pom/),
 which also lists every
 [managed plugin version](https://codehaus-plexus.github.io/plexus-pom/plugin-management.html).
 
 ## Requirements
 
-To build a project that inherits this POM, you need Java 8 or later and Maven 3.6.3 or later. To release
-one, you need Maven 3.9.0 or later.
+To build a project that inherits this POM, you need Java 8 or later and Maven 3.9.0 or later. Version 27
+raised the Maven minimum from 3.6.3; it applies to building the project, not to consuming its artifacts.
 
 ## Documentation
 

src/site/markdown/index.md

@@ -26,7 +26,7 @@ The following table lists the settings the parent POM fixes:
 | Setting | Value |
 |---|---|
 | Java, through the `javaVersion` property | 8, which sets `maven.compiler.source`, `target`, and `release` |
-| Minimum Maven to build | 3.6.3, or 3.9.0 to release |
+| Minimum Maven to build | 3.9.0, raised from 3.6.3 in version 27 |
 | Source encoding | UTF-8 |
 | Annotation processing | Off, through `maven.compiler.proc=none` |