Updated log4j and Checker Framework

Updating Checker Framework flagged numerous warnings, which have been
corrected. These probably involve type declarations lacking a nullness
annotation that appeared elsewhere.

build.gradle

@@ -22,10 +22,10 @@ allprojects {
         // dependency versions
         junitVersion = '5.3.0-M1'
         slf4jVersion = '1.7.25'
-        log4jVersion = '2.11.1'
+        log4jVersion = '2.11.2'
         nettyVersion = '4.1.34.Final'
         guavaVersion = '25.1-jre'
-        checkerFrameworkVersion = '2.5.6'
+        checkerFrameworkVersion = '2.7.0'
 
         getCurrentShortRevision = {
             new ByteArrayOutputStream().withStream { os ->