Add timestamp field to Maven POM when releasing

[Scrumplex]

Some users might want to know when a version of the loader was released. Prism Launcher currently tries to guess this by downloading all loader Jars from Quilt Maven and extracting the oldest timestamp present inside the archive.

An alternate approach to this would be to provide a timestamp in the Maven POM file.

The format allows for a properties field with arbitrary key-value mappings.[1] Additionally Gradle allows customizing the generated POM file before it is being pushed.[2]

For the sake of reproducibility, the timestamp should probably be of the last Git commit at build time.

See PrismLauncher/meta#18 for reasoning on why a stable release timestamp field would be beneficial.

Edit: eventually I would want this for other artifacts in Quilt Maven too, but Loader is the important one for now.

[TheGlitch76]

I'll look in to implementing this.

However, I can't do anything about older JARs. You'll have to find some way to statically store that information.