Add support for Maven 4

[dmikusa]

Describe the Enhancement

Maven 4 is in alphas. We will need to keep Maven 3 around for a while, so we should add Maven 4 to buildpack.toml so we can support both at the same time.

https://maven.apache.org/ref/4-LATEST/

Possible Solution

Add Maven 4 to buildpack.toml & adjust CI so that it tracks both major releases.

Motivation

Maven 4 is a new major branch & folks will likely want to use it.