Raise the minimum supported version of Gradle to 8.x

[wilkinsona]

If we add support for Gradle 9 (#43573) we should also consider raising the minimum supported version of Gradle to 8.x, dropping support for 7.6.x.

[wilkinsona]

Pushing back as it doesn't look like Gradle 9 is going to be released in time for 3.5.0-RC1.

[cpoissonnier]

It has been released since