Change the autobackport naming convention

Since the entire description of the original MR is used, it already contains the "Closes #XXXX" clause. If the branch would start with an issue number, then that clause would be automatically added to the backport description again, resulting in duplicate Closes entries in the MR description.

The simplest solution seems to be to use a different name convention for auto-backports to avoid this issue. If the backport branch doesn't start with an issue number, gitlab won't automatically modify the description.