Activate bom profile in log4j-bom

Adds a `.logging-parent-bom-activator` file to activate the `bom`
profile.

Author: ppkarwasz

.logging-parent-bom-activator

@@ -0,0 +1,16 @@
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+##
+This file activates the `flatten-bom` profile.

pom.xml

@@ -578,45 +578,6 @@
 
     <plugins>
 
-      <!-- Enable BOM flattening -->
-      <plugin>
-        <groupId>org.codehaus.mojo</groupId>
-        <artifactId>flatten-maven-plugin</artifactId>
-        <version>${flatten-maven-plugin.version}</version>
-        <executions>
-          <execution>
-            <id>flatten-revision</id>
-            <configuration>
-              <!--
-                ~ Flatten Maven Plugin removes the `child.project.url.inherit.append.path` attribute on this POM.
-                ~
-                ~ This causes a difference between the effective model of children built on this POM
-                ~ and those built on the flattened version.
-                ~
-                ~ Until https://github.com/mojohaus/flatten-maven-plugin/issues/412 we must expand &lt;url&gt;
-                -->
-              <pomElements>
-                <url>expand</url>
-              </pomElements>
-            </configuration>
-          </execution>
-          <execution>
-            <id>flatten-bom</id>
-            <goals>
-              <goal>flatten</goal>
-            </goals>
-            <phase>process-resources</phase>
-            <inherited>false</inherited>
-            <configuration>
-              <pomElements>
-                <!-- CycloneDX Maven Plugin uses this data -->
-                <distributionManagement>keep</distributionManagement>
-              </pomElements>
-            </configuration>
-          </execution>
-        </executions>
-      </plugin>
-
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