l10n: bump mshick/add-pr-comment from v2 to v3

l10n: bump mshick/add-pr-comment from v2 to v3

The l10n workflow uses `mshick/add-pr-comment` to post git-po-helper
reports as comments on translation pull requests. It was still pinned
to v2, which runs on Node.js 20. GitHub is phasing out the Node.js 20
runtime on Actions runners, so staying on v2 will eventually cause the
"Create comment in pull request for report" step to fail.

The sole breaking change in v3 is the switch from Node.js 20 to
Node.js 24 (https://github.com/mshick/add-pr-comment/releases/tag/v3.0.0).
The action's inputs and outputs are unchanged, so the upgrade is a
drop-in replacement. Subsequent v3.x releases added new opt-in
features (message truncation, retry with exponential backoff, file
attachments, commit comment support, "delete on status") but none of
them affect existing callers that do not opt in.

See also:

- Changelog: https://github.com/mshick/add-pr-comment/blob/main/CHANGELOG.md
- Compare: https://github.com/mshick/add-pr-comment/compare/v2...v3

Pointed-out-by: Christoph Grüninger <foss@grueninger.de>
Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>