[skip ci] UPGRADING: add missing RFC links and some more small fixes

[jrfnl]

No description provided.

[LamentXU123]

Not sure about the GMP part tho. Other commits looks correct.

[jrfnl]

@LamentXU123 Want me to move that commit to a separate PR (to unblock the other commits) ? Or do you want us to ping someone who was involved with the GMP change to review that commit ?

[jrfnl]

To clarify why the GMP change is as it is:

1. The move itself was inspired by a) seeing that ValueErrors were mostly listed in section 1 and b) seeing feedback comments asking to move the entries related to ValueErrors to section 1 on PRs related to other changes which introduced ValueErrors.
   It felt like giving that feedback was just overlooked/missed for the PR with the GMP change.
2. In the original file, line 65-66 and line 678-679 basically explained the same change.
3. I've merged the text of those four lines to come to what I felt was the most descriptive text to list that change.
4. The other entries which I moved from section 5 to section 1 are textually unchanged.
5. The order of the entries is kept the same as the order in which they were listed in section 5.

[LamentXU123]

I am not objecting it :) I just want to wait for other's opinion.

[jrfnl]

What can I do to move this forward ?

[LamentXU123]

Perhaps asking 8.6 RMs for final decisions on that file? @php/release-managers-86

[mbeccati]

LGTM

[LamentXU123]

Thanks!