Upgrade Gradle wrapper version to v7.3+ (support for Java 17)

[scottkurz]

It could be a good time to move the Gradle wrapper up to Gradle v7.x or even v8.x.

E.g. if Java 17 support were added (#482) it'd be a good time to move up to 7.3 at least (see: https://docs.gradle.org/current/userguide/compatibility.html).

As a side note (but related), just realized looking at this PR #483 that we can't upgrade the Liberty Gradle plugin used by the starter app beyond this version, since newer version don't have Gradle v6.x support.