Updating the migration guide for GHC 9.12

[Teofil Camarasu]

Hi folks,

With GHC 9.12 around the corner, I just wanted to give folks a gentle nudge
to continue filling out the 9.12 migration guide
<https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.12>.

Ideally we would have an entry for each change that can break user code.
And each entry would explain:
- what the change is
- what error messages caused by the change look like
- how users can fix their code

Though if you are aware of a breaking change and don't have the time to
fill out a full entry, a stub is also appreciated and I (or someone else)
can try to fill out the rest.

I was also wondering if we should better integrate the migration guide into
our processes. Maybe it should be mentioned in the MR template, so entries
are added as part of the MR flow?
I think that's the point when folks have stuff paged in and have the most
context, but I also don't want to add extra hoops to jump through. Any
thoughts?

Cheers,
Teo
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