feat(ci): add 'released' tag to container images from release runs (#11425)

## Problem
We had a problem with https://github.com/neondatabase/neon/pull/11413
having e2e tests failing, because an e2e test
(8d271bed47)
depended on an unreleased pageserver fix
(0ee5bfa2fc).
This came up because neon release CI runs against the most recent
releases of the other components, but cloud e2e tests run against
latest, which is tagged from main.

## Summary of changes
Add an additional `released` tag for released versions.

## Alternative to consider
We could (and maybe should) instead switch to `latest` being used for
released versions and `main` being used where we use `latest` right now.
That'd also mean we don't have to adjust the CI in the cloud repo.

.github/scripts/generate_image_maps.py

@@ -39,12 +39,18 @@ registries = {
     ],
 }
 
+release_branches = ["release", "release-proxy", "release-compute"]
+
 outputs: dict[str, dict[str, list[str]]] = {}
 
-target_tags = [target_tag, "latest"] if branch == "main" else [target_tag]
-target_stages = (
-    ["dev", "prod"] if branch in ["release", "release-proxy", "release-compute"] else ["dev"]
+target_tags = (
+    [target_tag, "latest"]
+    if branch == "main"
+    else [target_tag, "released"]
+    if branch in release_branches
+    else [target_tag]
 )
+target_stages = ["dev", "prod"] if branch in release_branches else ["dev"]
 
 for component_name, component_images in components.items():
     for stage in target_stages: