Update dependency resolution management (#2000)

## Changes
* get rid of the JCenter repository
* centralize repository declaration for all projects in project settings
* refactor `build.gradle` and `settings.gradle` from Groovy syntax to
Kotlin DSL (thus replacing those files with `build.gradle.kts` and
`settings.gradle.kts` respectively)

The remaining `build.gradle` files are refactored in:
https://github.com/syncthing/syncthing-android/pull/2022

build.gradle.kts

@@ -1,32 +1,27 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
-    ext {
+    extra.apply {
         // Cannot be called "ndkVersion" as that leads to naming collision
         // Changes to this value must be reflected in `./docker/Dockerfile`
-        ndkVersionShared = '25.2.9519653'
+        set("ndkVersionShared", "25.2.9519653")
+
     }
+
     repositories {
+        gradlePluginPortal()
         google()
-        jcenter()
+        mavenCentral()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:7.3.1'
-        classpath 'com.github.ben-manes:gradle-versions-plugin:0.36.0'
+        classpath("com.android.tools.build:gradle:7.3.1")
+        classpath("com.github.ben-manes:gradle-versions-plugin:0.36.0")
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }
 }
 
-allprojects {
-    repositories {
-        google()
-        jcenter()
-        maven { url = "https://jitpack.io" }
-    }
-}
-
-task clean(type: Delete) {
-    delete rootProject.buildDir
+tasks.register<Delete>("clean") {
+    delete(rootProject.buildDir)
 }

settings.gradle

@@ -1 +0,0 @@
-include ':app', ':syncthing'

settings.gradle.kts

@@ -0,0 +1,21 @@
+pluginManagement {
+    repositories {
+        gradlePluginPortal()
+        google()
+        mavenCentral()
+    }
+}
+
+dependencyResolutionManagement {
+    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
+    repositories {
+        google()
+        mavenCentral()
+        maven { url = java.net.URI("https://jitpack.io") }
+    }
+}
+
+include(
+    ":app",
+    ":syncthing"
+)