Update Gradle wrapper to 5.6 (#26079)

Summary:
```
Welcome to Gradle 5.6!

Here are the highlights of this release:
 - Incremental Groovy compilation
 - Groovy compile avoidance
 - Test fixtures for Java projects
 - Manage plugin versions via settings script

For more details see https://docs.gradle.org/5.6/release-notes.html
```

## Changelog

[Android] [Changed] - Gradle wrapper 5.6
Pull Request resolved: https://github.com/facebook/react-native/pull/26079

Test Plan: Ran build and tests locally.

Differential Revision: D17054310

Pulled By: mdvacca

fbshipit-source-id: de7ba3a6d04058e51b8bc6a21d5a3f828ef8bc25