build: update 'proguard' from 6.1.1 to 7.0.0

`jcenter()` it's needed because they didn't upload to Maven Central yet. https://github.com/Guardsquare/proguard/issues/54

There are some minor issues with proguard still.
It produces a lot of warnings since Gradle 6.0, but it's fixable on their side. https://github.com/Guardsquare/proguard/issues/14

brut.apktool/apktool-cli/build.gradle

@@ -23,10 +23,13 @@ dependencies {
 buildscript {
     repositories {
         mavenCentral()
+        jcenter()
     }
 
     dependencies {
-        classpath depends.proguard_gradle
+        classpath(depends.proguard_gradle) {
+            exclude group: 'com.android.tools.build'
+        }
     }
 }