upgrade to log4j2

Upgraded dependencies in Grouper projects to log4j2 (just the 2.6 branch until testing shows no problems).

Since the underlying engine is now log4j2, you need to either convert your log4j.properties to a log4j2 compatible format, or pass in JVM argument `-Dlog4j1.compatibility=true` (e.g. in GSH_JVMARGS, GROUPER_EXTRA_CATALINA_OPTS). The container image will be adjusted to add this by default in a separate commit.