Work around JENKINS-68568 (#563)

pom.xml

@@ -958,6 +958,49 @@
         <!-- Filled in by maven-hpi-plugin from the MANIFEST.MF entry in jenkins.war, but we provide a default value for the benefit of IDEs. -->
         <jenkins.addOpens>--add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.io=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED</jenkins.addOpens>
       </properties>
+      <!--
+        Maven HPI plugin 3.27 or later (which is included in plugin parent POM 4.40 or later) almost
+        allows the plugin POM to support cores that require both Java 8 and Java 11 by dynamically
+        reconfiguring "maven.compiler.release" and "maven.compiler.testRelease" (which default to 8
+        in order to support cores prior to 2.357) to the version of Java that
+        "jenkins/model/Jenkins.class" was compiled with (which could be 11 with 2.357 or later):
+        "almost" because Enforcer's bytecode version check still gets in the way if a plugin has a
+        baseline of 2.357 or later and has a library on its classpath with Java 11 bytecode.
+        Unfortunately, there is no easy way for Maven HPI plugin to dynamically reconfigure Enforcer
+        for Java 11 the same way it dynamically reconfigures Maven Compiler Plugin for Java 11. So in
+        order for the plugin POM to support baselines before and after 2.357 and allow plugins to
+        use Java 11 features with a baseline of 2.357 or later if so desired, we work around the
+        problem by increasing the maximum JDK version for Enforcer to 11 below. While technically
+        incorrect (in the sense that it fails to enforce the absence of Java 9, 10, or 11 bytecode
+        when it should) when building, testing, or releasing a plugin with a baseline prior to 2.357
+        using a Java 11 compiler (including CD), any such plugin should have a "Jenkinsfile" build
+        running on Java 8, where the correct enforcement should be present, so this should not cause
+        any practical problems. This workaround is justified because the alternative (raising the
+        minimum Jenkins version for the plugin POM to 2.357 or later and dropping support for Java
+        8) would be overly harsh on users, causing Dependabot updates to fail for the majority of
+        users who do not want to update their baseline so aggressively. When we are ready to raise
+        the minimum Jenkins version for the plugin POM to 2.357 or later and drop support for Java
+        8, this workaround can be deleted.
+      -->
+      <build>
+        <plugins>
+          <plugin>
+            <artifactId>maven-enforcer-plugin</artifactId>
+            <executions>
+              <execution>
+                <id>display-info</id>
+                <configuration>
+                  <rules>
+                    <enforceBytecodeVersion>
+                      <maxJdkVersion>11</maxJdkVersion>
+                    </enforceBytecodeVersion>
+                  </rules>
+                </configuration>
+              </execution>
+            </executions>
+          </plugin>
+        </plugins>
+      </build>
     </profile>
     <profile>
       <id>jenkins-release</id>