compile and run against java 21 LTS

[johnduffell]

Further to updating to arm64 architecture #2194

This PR updates to java 21 LTS. This seems to have bumped performance significantly with support-workers guardian/support-frontend#5792 , so I am replicating it here.

I have also bumped the version we compile with in GHA, in case that improves compilation performance.

I tested auto cancel in CODE and it seems to have improved the cold start noticably

[johnduffell]

this parameter was causing a lot of warnings, also it wasn't really relevant any more. It would only be needed if we were compiling on a later VM and running on an earlier one.

[johnduffell]

I've seen problems locally where people have been on a later version than is supported by either SBT/play/scala, so I have kept this in and updated it.
If it's causing issues locally we can rethink.

[rtyley]

Looks like we need to bump the version of Java used by our Scala Steward workflow as well:

https://github.com/guardian/scala-steward-public-repos/actions/runs/8278707245/job/22651625930

java.lang.AssertionError: assertion failed: Java 21 is required for this project, download corretto-21 from File->Project structure.
at scala.Predef$.assert(Predef.scala:223)
at $cd99ddf78d62fbdae579$.$anonfun$$sbtdef$1(/home/runner/scala-steward/workspace/repos/guardian/support-service-lambdas/build.sbt:863)
at $cd99ddf78d62fbdae579$.$anonfun$$sbtdef$1$adapted(/home/runner/scala-steward/workspace/repos/guardian/support-service-lambdas/build.sbt:859)
at scala.Function1.$anonfun$compose$1(Function1.scala:49)

[johnduffell]

good spot @rtyley I have raised a PR to update scala steward, however it is likely to fail on other projects where they are not up to recent versions of sbt, it's needing 2.12.18 or later to work https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html