Allow developer control of parallel testing

Move the definition of parallel testing from the Maven pom file into the
Jenkinsfile so that ci.jenkins.io continues to run the tests with one
process per available core, while developers are allowed to configure
the amount of parallel testing based on the configuration and use of
their computer.

Developers can adjust parallel execution by passing a command line
argument to Maven like this:

  mvn clean -DforkCount=1C verify

Developers can define a Maven profile that sets the forkCount in their
~/.m2/settings.xml like this:

  <profile>
    <id>faster</id>
    <activation>
      <activeByDefault>true</activeByDefault>
    </activation>
    <properties>
      <forkCount>.45C</forkCount>
    </properties>
  </profile>

With that entry in the settings.xml file, then 0.45C will be used for:

  mvn clean verify

Jenkinsfile

@@ -1,4 +1,4 @@
-buildPlugin(useContainerAgent: true, configurations: [
+buildPlugin(useContainerAgent: true, forkCount: '1C', configurations: [
 [platform: 'linux', jdk: '11'],
 [platform: 'linux', jdk: '17'],
 ])

pom.xml

@@ -110,20 +110,6 @@
         </dependency>
     </dependencies>
 
-    <build>
-        <plugins>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-surefire-plugin</artifactId>
-                <configuration>
-                    <parallel>all</parallel>
-                    <useUnlimitedThreads>true</useUnlimitedThreads>
-                    <forkCount>1C</forkCount>
-                </configuration>
-            </plugin>
-        </plugins>
-    </build>
-
     <repositories>
       <repository>
         <id>repo.jenkins-ci.org</id>