config.yaml: Bump aum offline upgrade branch test release to latest rc

After bumping the release used for the aum offline upgrade branch tests
in aabc0578 to v2023.3 it became clear that this was not enough.  v2023.3
was released in November 2023, the required update to the partition
layout wasn't pushed back to the v2023 branch until March 2024. As a
result the current releases are broken.

As we now have an rc release that should contain the required fix,
switch to using this to ensure that this is indeed rectifies the issue.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

lava/config.yaml

@@ -9,7 +9,7 @@ variables:
   # Any release listed here MUST be in the OSTree ref-bindings in apertis-image-recipes:
   # https://gitlab.apertis.org/infrastructure/apertis-image-recipes/-/blob/apertis/v2025dev2/ostree-commit.yaml
   upgradable_release: "v2023"
-  upgradable_build: "3"
+  upgradable_build: "5rc1"
   arch: amd64
   visibility: public
   priority: medium