Upgrade to Gradle 6.8.1

[wilkinsona]

When upgrading, we should also update the Gradle plugin's compatibility testing so that the tests continue to cover the version of Gradle that we're using at the moment (6.7.1).

[wilkinsona]

Upgrading to 6.8 fails due to this regression in Gradle. We could avoid the problem by writing out the file with malformed unicode content rather than storing it in src/test/resources but I'd prefer to wait for a fix on the Gradle side instead.