Change default alpine flavor to point to alpine 3.19

Status update. (2024-08-16)

• The default runner alpine version was updated in Fix default runner alpine flavour version (!4850 - merged) (in %17.2) to address https://gitlab.com/gitlab-org/gitlab-runner/-/issues/37654#note_1969160620. https://gitlab.com/gitlab-org/gitlab-runner/-/blob/179eeeb3cc559b6f10d6b5d6a801610a94b0be44/docs/configuration/advanced-configuration.md#L1090-1090

Update docs and a few variables so the default alpine image points to the latest.

We do this only on major releases