Bump maven-compiler-plugin to 3.12.1

https://issues.apache.org/jira/secure/ReleaseNote.jspa?projectId=12317225&version=12354061

JIRA: LIGHTY-283
Signed-off-by: tobias.pobocik <tobias.pobocik@pantheon.tech>

Author: Tobianas

lighty-core/lighty-parent/pom.xml

@@ -79,7 +79,7 @@
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-compiler-plugin</artifactId>
-                    <version>3.11.0</version>
+                    <version>3.12.1</version>
                 </plugin>
                 <plugin>
                     <groupId>org.apache.maven.plugins</groupId>