feat: upgrade gradle to 6.1 & gradle build tools to 3.5.3 (#792)

* feat!: upgrade gradle to 6.1
* feat!: upgrade gradle build tools to 3.5.3
* feat: added `npm run clean-tests`
* fix!: Removed useDeprecatedNdk as this option is now completely removed from gradle.
* feat!: bump gradle to 6.1 & gradle build tools to 3.5.3

Co-authored-by: エリス <erisu@users.noreply.github.com>

bin/templates/cordova/lib/config/GradlePropertiesParser.js

@@ -34,10 +34,7 @@ class GradlePropertiesParser {
             'org.gradle.daemon': 'true',
 
             // to allow dex in process
-            'org.gradle.jvmargs': '-Xmx2048m',
-
-            // allow NDK to be used - required by Gradle 1.5 plugin
-            'android.useDeprecatedNdk': 'true'
+            'org.gradle.jvmargs': '-Xmx2048m'
 
             // Shaves another 100ms, but produces a "try at own risk" warning. Not worth it (yet):
             // 'org.gradle.parallel': 'true'