Upgrade to Gradle 7.4.2

The latest version of Gradle is 7.4.2. The mainClassName property has
been deprecated and replaced with the mainClass property. The latest
shadow version compatible with Gradle 7.4.2 is 7.1.2.

Let's upgrade to Gradle 7.4.2, resolve the deprecation and upgrade to
shadow 7.1.2.

build.gradle

@@ -2,7 +2,7 @@ plugins {
     id 'java'
     id 'application'
     id 'checkstyle'
-    id 'com.github.johnrengelman.shadow' version '6.1.0'
+    id 'com.github.johnrengelman.shadow' version '7.1.2'
 }
 
 repositories {
@@ -29,7 +29,7 @@ test {
 }
 
 application {
-    mainClassName = "seedu.duke.Duke"
+    mainClass = "seedu.duke.Duke"
 }
 
 shadowJar {

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.9.2-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.4.2-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists