contrib: Fix start-release "old release" detection
contrib: Fix start-release "old release" detection

This script previously assumed that you first checkout the branch where
you will make the release, then run the script. By first pulling &
creating the branch in the script, we can detect the "old_version"
correctly with fewer dependencies on the release manager's environment.

Reported-by: Chris Tarazi <711c73f64afdce07b7e38039a96d2224209e9a6c@isovalent.com>
Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium