Stop testing against Gradle 7.6 RCs

[wilkinsona]

Since we started testing against the Gradle 7.6 RCs we've seen a marked increase in test flakiness. The flaky failures are due to too many open files during compilation. Our suspicion is that there's a file handle leak in Gradle 7.6. Another possibility is that there's a leak elsewhere and adding Gradle 7.6 to the test matrix is the straw that's broken the camel's back. We'll investigate, but for now we need to re-stabilise the build.