Bump spotbugs-maven-plugin to 4.8.6.4

https://github.com/spotbugs/spotbugs-maven-plugin/releases/tag/spotbugs-maven-plugin-4.8.6.3
https://github.com/spotbugs/spotbugs-maven-plugin/releases/tag/spotbugs-maven-plugin-4.8.6.4

JIRA: LIGHTY-332
Signed-off-by: tobias.pobocik <tobias.pobocik@pantheon.tech>

Author: Tobianas

lighty-core/lighty-parent/pom.xml

@@ -210,7 +210,7 @@
                 <plugin>
                     <groupId>com.github.spotbugs</groupId>
                     <artifactId>spotbugs-maven-plugin</artifactId>
-                    <version>4.8.6.1</version>
+                    <version>4.8.6.4</version>
                     <dependencies>
                         <dependency>
                             <groupId>com.github.spotbugs</groupId>