Bug 1802817 - Replace deprecated main with mainClass.

See https://docs.gradle.org/7.6/dsl/org.gradle.api.tasks.JavaExec.html#org.gradle.api.tasks.JavaExec:main.

build.gradle

@@ -240,7 +240,7 @@ tasks.register('ktlint', JavaExec) {
 task ktlintFormat(type: JavaExec, group: "formatting") {
     description = "Fix Kotlin code style deviations."
     classpath = configurations.ktlint
-    main = "com.pinterest.ktlint.Main"
+    mainClass.set("com.pinterest.ktlint.Main")
     args "-F", "app/src/**/*.kt", "!**/build/**/*.kt", "--baseline=ktlint-baseline.xml"
 }