Improve issue deduplication strategy for generate-releases

[alice-i-cecile]

Non-blocking: Are we concerned about titles being possibly renamed? I doubt that would happen very often (if at all), but it might be somewhat difficult if we ever want to change the title format to something else.

This might be better served as a followup, but I've seen people use hidden comments at the start of the issue body to contain metadata about the issue. That might make any future migrations a bit easier.

Originally posted by @MrGVSV in #1222 (comment)

[alice-i-cecile]

As discussed there, the most robust thing to do is to probably carefully parse the linked PR number back out and compare that.