Update gradle to v6.8.1

Summary: Fixes regressions in v6.8.0. Full list of fixed issues is available here.

Test Plan: Build a Gradle project from source

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D10415