Make the default helper alpine flavour point to latest

Over in Update/Create new gitlab runners to support bun... (gitlab-com/gl-infra/production-engineering#26837) the runners-platform folks are asking for us to change the default helper alpine flavour tag from 3.21 to latest. They want this so they can take advantage of some performance optimizations GitLab contributed to git. Those optimization changes were first made available in git v2.49.0; taht git version is available in alpine versions 3.22 or newer. Our latest helper alpine version flavour currently points to 3.22. You might think "why don't they use the 3.22 tag directly?" If you recall, we're in the process of moving away from tagging multiple helper alpine flavour versions, and will eventually have only a latest tag, which will reflect upstream's latest. So the only way to use the 3.22 is via the latest tag. You might also think "Why don't they explicitly use the latest flavour tag?". To do that they'd have to update all their runner configurations, which is not small job.

Once alpine 3.19 and 3.21 are EOL and we drop support for them, default and latest will be the same thing, so this is a change that would happen anyway eventually. We're just doing it sooner.