Switch giant to using java 21

[philmcmahon]

What does this change?

There's no real urgency with this change as corretto 11 is supported until 2032 but I wanted to add https://github.com/guardian/play-brotli-filter to giant, and that only supports java 21, so I'm moving giant onto java 21.

Changes:

• Replace UseConcMarkSweepGC with UseG1GC - this has been recommended since java 9 - see https://openjdk.org/jeps/291
• RAMFraction becomes RAMPercentage - RAMFraction still works but is deprecated

Getting this to work locally: this depends how you're managing java on your machine. I use https://github.com/jenv/jenv so I had to do:

• brew install --cask corretto@21
• jenv add /Library/Java/JavaVirtualMachines/amazon-corretto-21.jdk/Contents/Home
• after this cding into the giant directory should get jenv to pick the right version - check with jenv versions

Note - this can't be released until we've fixed our AMI bakes

How to test

I've tested this on playground using this AMI: https://amigo.gutools.co.uk/recipes/investigations-giant-app-arm-j21/bakes/1

[twrichards]

LGTM (we're so 'cutting edge')