Upgrade maven

Versions before 3.2.3 don't use https when accessing Maven Central,
which is now required (see MNG-5672).

Also upgrade findbugs to a version compatible with newer Maven versions.

.mvn/wrapper/maven-wrapper.properties

@@ -1 +1 @@
-distributionUrl=https://repo1.maven.org/maven2/org/apache/maven/apache-maven/3.1.1/apache-maven-3.1.1-bin.zip
+distributionUrl=https://repo1.maven.org/maven2/org/apache/maven/apache-maven/3.6.3/apache-maven-3.6.3-bin.zip

pom.xml

@@ -160,7 +160,7 @@
 				<plugin>
 					<groupId>org.codehaus.mojo</groupId>
 					<artifactId>findbugs-maven-plugin</artifactId>
-					<version>3.0.1</version>
+					<version>3.0.5</version>
 				</plugin>
 				<plugin>
 					<groupId>org.sonatype.plugins</groupId>