docs/relnotes: update internal references

I'm not 100% sure if it feels right to update these. I mean, this keeps
links working as they should, even if exported to something else than
HTML. But it also feels a bit like history revisionism. It's probably
the right thing to do, though.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>
36 files changed