math/mpir: Fix upstream site

Apparently, the old domain no longer belongs to the upstream.
However, the source code is available on GitHub.

While here pet portclippy.

PR: 294784
Approved by: wen (maintainer)
Sponsored by: UNIS Labs
MFH: 2026Q2

(cherry picked from commit 4446355417e101af1680a69aab8a181b90faa664)