.github: Update release template for RCs
.github: Update release template for RCs

The instructions for release candidate releases are slightly adjusted
from a regular release. Add them into the main template that we use for
most releases, now that the scripts have been updated to support this
release type.

These steps also largely apply the same way for a release with the
format "vX.Y.Z-snapshotN".

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

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