Upgrade to Gradle 6.8.1 (#1895)
Upgrade to Gradle 6.8.1 (#1895)

Summary:
Playing around with alternative upload options and having
an up-to-date Gradle installation makes that easier.

Pull Request resolved: https://github.com/facebook/flipper/pull/1895

Test Plan: ./gradlew :sample:assembleDebug + CI

Reviewed By: nikoant

Differential Revision: D26275350

Pulled By: passy

fbshipit-source-id: ce5d11c289e6cf52e2d629a4719cc38665adf8e5

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