Bump maven-compiler-plugin to 3.15.0

https://github.com/apache/maven-compiler-plugin/releases/tag/maven-compiler-plugin-3.15.0

Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
(cherry picked from commit 9aa1a04)

Author: ihrasko

lighty-core/lighty-parent/pom.xml

@@ -78,7 +78,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-compiler-plugin</artifactId>
-                    <version>3.14.1</version>
+                    <version>3.15.0</version>
                 </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>