[Java] Upgrade to Gradle 4.8.1.

Author: mjpt777

build.gradle

@@ -701,7 +701,7 @@ task uploadToMavenCentral {
         'sbe-all:uploadShadow'
 }
 
-task wrapper(type: Wrapper) {
-    gradleVersion = '4.7'
+wrapper {
+    gradleVersion = '4.8.1'
     distributionType = 'ALL'
 }

gradle/wrapper/gradle-wrapper.jar

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.7-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.8.1-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists