Scala Steward can not run on this repo until it allows building under Java 21

[rtyley]

Following on from guardian/scala-steward-public-repos#67 ... we're now running our Scala Steward workflow with Java 21 LTS, which means that all projects that want to have Scala Steward updates (like this one) need to be able to build under Java 21 (even if the projects are still running in production on Java 8).

An example of Scala Steward failing when trying to handle this repo is here:

https://github.com/guardian/scala-steward-public-repos/actions/runs/9114552795/job/25058784453#step:6:2202

Unfortunately the Scala Steward run failing affects everyone, see guardian/scala-steward-public-repos#60 - so I've removed this repo from Scala Steward until this issue is fixed.

How to fix this issue?

Update Scala (2.12.18+) & sbt (1.9.0+)!

cc @lindseydew !