build: Update release branch name (openedx#31896)

This is part of edx/edx-arch-experiments#212.
2U is switching to using a prefix to reflect edx-platform's position as a
community repo. Our deployment process will also no longer produce
per-deploy branches and tags. This one branch will remain in place rather
than being kept in a private repo since the named-release branch-cutting
process still uses this as an indicator of what code has been field-tested.

openedx.yaml

@@ -2,7 +2,7 @@
 # https://open-edx-proposals.readthedocs.io/en/latest/oep-0002-bp-repo-metadata.html#specification
 
 nick: edx
-openedx-release: {ref: release}
+openedx-release: {ref: "2u/release"}
 
 oeps:
     oep-2: true