Update apps generator plugin dependencies

This commit updates all library and plugin dependencies
in the `spring-cloud-dataflow-apps-plugin` modules to
the latest versions.

Additionally:
- performs a general cleanup of build and README files.
- updates to maven 3.9.9

Resolves #498

* Address code review comments:

- Use `1.1.0-SNAPSHOT` as version in metadata plugin README.adoc
  to be consistent w/ other examples

spring-cloud-dataflow-apps-plugin/.mvn/jvm.config

@@ -1 +1 @@
--Xmx1024m -XX:CICompilerCount=1 -XX:TieredStopAtLevel=1 -Djava.security.egd=file:/dev/./urandom
+-Xmx1024m -XX:CICompilerCount=1 -XX:TieredStopAtLevel=1 -Djava.security.egd=file:/dev/./urandom --add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED

spring-cloud-dataflow-apps-plugin/.mvn/maven.config

@@ -1 +0,0 @@
--DaltSnapshotDeploymentRepository=repo.spring.io::default::https://repo.spring.io/libs-snapshot-local -P spring

spring-cloud-dataflow-apps-plugin/.mvn/wrapper/maven-wrapper.properties

@@ -1,2 +1,20 @@
-distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.6.3/apache-maven-3.6.3-bin.zip
-wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar
+# 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.
+wrapperVersion=3.3.2
+distributionType=bin
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.9.9/apache-maven-3.9.9-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/org/apache/maven/wrapper/maven-wrapper/3.3.2/maven-wrapper-3.3.2.jar

spring-cloud-dataflow-apps-plugin/etc/checkstyle/checkstyle-suppressions.xml

@@ -4,6 +4,7 @@
 		"https://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
 <suppressions>
 	<suppress files="(.*EmbeddedEngine\.java)" checks="[a-zA-Z0-9]*"/>
+	<suppress files="(.*EmbeddedEngine2\.java)" checks="[a-zA-Z0-9]*"/>
 	<suppress files="(.*Class\.java)" checks="[a-zA-Z0-9]*"/>
 	<suppress files="[\\/]src[\\/]main[\\/](protos)[\\/]"
 			  checks="(Javadoc*|HideUtility*)"/>

spring-cloud-dataflow-apps-plugin/etc/checkstyle/checkstyle.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <!DOCTYPE module PUBLIC
-		"-//Puppy Crawl//DTD Check Configuration 1.3//EN"
-		"https://www.puppycrawl.com/dtds/configuration_1_3.dtd">
+		"-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+		"https://checkstyle.org/dtds/configuration_1_3.dtd">
 <module name="com.puppycrawl.tools.checkstyle.Checker">
 
 	<module name="SuppressionFilter">
@@ -20,9 +20,8 @@
 		<property name="headerFile" value="${checkstyle.header.file}"/>
 		<property name="fileExtensions" value="java,groovy"/>
 	</module>
-	<module name="com.puppycrawl.tools.checkstyle.checks.NewlineAtEndOfFileCheck">
-		<property name="lineSeparator" value="lf"/>
-	</module>
+
+	<module name="NewlineAtEndOfFile"/>
 
 	<!-- TreeWalker Checks -->
 	<module name="com.puppycrawl.tools.checkstyle.TreeWalker">