Make Spark's guava version configurable from the maven command line.

All future releases of hadoop are going to ship with 27.0 or later, *including point releases of the 3.1 branch, which is a mixed blessing.

Pro:

• it's up to date
• no active CVEs

Con:

• code which uses things the guava team removed won't compile
• code built with the later release won't link to older versions due to signature overloading, with Preconditions.checkArgument a key example

Making the guava.version spark pulls in an overrideable value makes it possible to choose the version to build with alongside the -Dhadoop.version selection of hadoop version.