Convert Velocity buildscripts to Kotlin DSL (#918)

Spiritually indebted to #518 and @alexstaeding.

There's a minor break - we're going up to 3.2.0-SNAPSHOT as the API now compiles against Java 11. But this is more academic in practice.

gradle/checkstyle.gradle

@@ -1,8 +0,0 @@
-checkstyle {
-    toolVersion '10.3.1'
-    configFile new File(project.rootDir, ['config', 'checkstyle', 'checkstyle.xml'].join(File.separator))
-    
-    // The build should immediately fail if we have errors.
-    maxErrors = 0
-    maxWarnings = 0
-}

gradle/publish.gradle

@@ -1,13 +0,0 @@
-publishing {
-    repositories {
-        maven {
-            credentials(PasswordCredentials.class)
-
-            name = 'paper'
-            def base = 'https://papermc.io/repo/repository/maven'
-            def releasesRepoUrl = "$base-releases/"
-            def snapshotsRepoUrl = "$base-snapshots/"
-            url = version.endsWith('SNAPSHOT') ? snapshotsRepoUrl : releasesRepoUrl
-        }
-    }
-}