Small bits of cleanup

Build system:
 - Switch to our new maven server. This has a cleaner separation between
   published packages and mirrored packages, to avoid leaking those into
   other people's builds.
 - Update Gradle and Loom versions.

Code:
 - Link to definitions instead in the breaking changes page.
 - Fix several unused variable warnings.

Other:
 - Remove unsupported Minecraft versions from the issue template.

buildSrc/src/main/kotlin/cc-tweaked.java-convention.gradle.kts

@@ -38,7 +38,7 @@ java {
 repositories {
     mavenCentral()
 
-    val mainMaven = maven("https://squiddev.cc/maven") {
+    val mainMaven = maven("https://maven.squiddev.cc/mirror") {
         name = "SquidDev"
     }
 
@@ -133,8 +133,8 @@ tasks.processResources {
 tasks.withType(AbstractArchiveTask::class.java).configureEach {
     isPreserveFileTimestamps = false
     isReproducibleFileOrder = true
-    dirMode = Integer.valueOf("755", 8)
-    fileMode = Integer.valueOf("664", 8)
+    filePermissions {}
+    dirPermissions {}
 }
 
 tasks.jar {