gradle: upgrade to 6.3

It looks like F-Droid finally updated.

This reverts commit cd43444d1f.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.2.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists