build: clone libxml2 from the GitHub mirror (#2033)

## Summary

The libxml2 clone from `gitlab.gnome.org` failed twice with HTTP 503
when GNOME's Gitaly backend was unavailable, breaking the manylinux
wheel build mid-way:

| Build | Date | Error |
|---|---|---|
| #997 | Jul 13 | `Gitaly is not available` / HTTP 503 |
| #1254 | Jul 31 | `Gitaly is not available` / HTTP 503 |

Clone from the GitHub mirror instead, which is not affected by GNOME
GitLab availability.

## Changes

- **`contrib/Dockerfile.manylinux`** - point the libxml2 clone at
`https://github.com/GNOME/libxml2.git`
- **`ATTRIBUTIONS-CPP.md`** - correct the libxml2 version from 2.9.14 to
2.15

Everything else about the clone step is unchanged: same `--depth 1`,
same `2.15` branch, same build commands.

The attribution recorded 2.9.14 while the Dockerfile has been building
the 2.15 branch. It is recorded as `2.15` rather than a patch version
because the clone tracks the branch rather than a tag.

---------

Signed-off-by: NirWolfer <nwolfer@nvidia.com>