build: Clean up repository configuration

- Put jitpack.io at the bottom, so that it is the last one checked. For libraries that are both in jitpack and elsewhere,
this should prevent a lot of errors especially in CI due to jitpack unreliability.
- Don't use gradle plugin portal as a mirror for JCenter. JCenter is still up as read-only, and using the gradle plugin portal
hides the fact that we have stop using jcenter libraries.
- For buildscripts, use gradlePluginPortal() instead of manual maven config
- Don't have repository configuration in both app/build.gradle and project build.gradle. Use project gradle instead

Signed-off-by: Álvaro Brey Vilas <alvaro.brey@nextcloud.com>

app/build.gradle

@@ -54,15 +54,6 @@ configurations.configureEach {
     }
 }
 
-repositories {
-    google()
-    maven { url "https://jitpack.io" }
-    mavenCentral()
-    maven {
-        url 'https://plugins.gradle.org/m2/'
-    }
-}
-
 // semantic versioning for version code
 def versionMajor = 3
 def versionMinor = 20

build.gradle

@@ -26,17 +26,16 @@ buildscript {
 subprojects {
     buildscript {
         repositories {
+            gradlePluginPortal()
             google()
-            maven {
-                url 'https://plugins.gradle.org/m2/'
-            }
             mavenCentral()
         }
     }
     repositories {
         google()
-        maven { url "https://jitpack.io" }
         mavenCentral()
+        jcenter()
+        maven { url "https://jitpack.io" }
     }
 }