Release oldstable from v3, edge from v4

[DrJosh9000]

Description

Create a new "codename" oldstable (like Debian's oldstable), and release v3 to it.
Also push to edge from v4 instead of v3, so that we can dogfood v4 more.

Context

v4 is coming - #3807

Changes

• Create an oldstable release pipeline.yml. (Depends on creating the pipeline and setting up the OIDC permissions ✅ Done)
• Update scripts to accept oldstable as a codename.
• Only push version tags (e.g. 3, 3.123) as container image tags from oldstable, and not e.g. latest
• Various shell style tweaks in touched files, mainly $FOO -> ${FOO} and changing consecutive if...fi blocks into case...esac

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Disclosures / Credits

I did not use AI tools at all