Upgrade to JUnit 5

From https://lists.apache.org/thread/ygtkpch0s7nss4vx8xfytwsbnv7mzss9

Matt Juntunen - Monday, 4 July 2022 04:40:43 CEST
Re: [VOTE] Release Apache Commons Configuration 2.8.0 based on RC3
Thanks, Bruno and Gary!

Gary,
I think the unit tests as a whole could use some upgrading. I'm
picturing a conversion to JUnit 5, during which we could ensure
compatibility with the latest JDK versions. I'm not sure what to do
about the javadoc warnings on the generated classes. From the comments
in the pom, this seems to have been an issue since v2.4.

Regards,
Matt J