config: Bump latest branch to latest3

Per endlessm/eos-ostree-builder@45fd862, the eos6.0 ostree builds are
now pushing to a `latest3` branch. eos6.0 image builds have been broken
since that change.

https://phabricator.endlessm.com/T35243

config/defaults.ini

@@ -316,7 +316,7 @@ remote = eos
 # For LTS builds this may be the built branch (e.g., eos4.0) or the
 # major series branch (e.g., eos4) to keep the system on a specific EOS
 # version.
-stable_branch = latest2
+stable_branch = latest3
 ref_deploy = os/${product}/${platform}/${stable_branch}
 
 # Builder directories