Reduce number of supported alpine versions to just latest

Overview

We still plan to drop all explicitly versioned alpine images and keep only a latest image, but we will do this gradually so it's not a breaking change. Instead of dropping all versioned alpine image in %18.0, we'll let them age out naturally (when the images es EOLed upstream), and not add any new explicitly versioned images; only latest.

Additional background

Current when creating runner/helper images, we create alpine-based images for the following versions:

• 3.16
• 3.17
• 3.18
• 3.19

Where latest == 3.19

With Update alpine versions (!5214 - merged) that changes to

• 3.18
• 3.19
• 3.21

Where latest == 3.21.