Raise the minimum supported version of Gradle to 7.6.4+ or 8.3+

[wilkinsona]

We currently support 7.x (7.5 or later) and 8.x. This causes us some problems as not all of these versions of Gradle work when a plugin or one of its dependencies is a multi-release jar. Support for such multi-release jars was added in a 7.6.x patch release and 8.3 so I think we should support 7.x (7.6.4 or later) and 8.x (8.3 or later).

Depending on when Gradle 9 is released, we may want to revisit this later in the 3.4 development cycle and drop support for 7.x entirely.