Issue templates: updated to better fit actual practice

The `gitlab-prebuilds` job is a pain, and not these days reliably
launched as part of the MR pipeline. Of course it does not complete before
the first job in the main pipeline needs it…

branch : heptapod
12 jobs for !314 with topic/heptapod/release-task-lists in 2 minutes and 19 seconds (queued for 3 seconds)
latest merge request