Sphinx can be a bit overly eager at interpreting text with colons in it
as URLs. There's a few cases in our older relnotes where this happens, so
let's escape them to avoid strange, broken links in the rendered output.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19574>