Closes #1789.
The bug was due to the fact that GitLab does not guarantee that issue numbers are created sequentially: some identifiers can be skipped. Therefore, the new pull requests numbers should not be offset by the number of issues, but by the maximum issue number.
See for instance https://gitlab.com/troyengel/archbuild/-/issues/?sort=created_date&state=all&first_page_size=20, where there is only a singe issue with number "2".
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/1790
Co-authored-by: Antonin Delpeuch <antonin@delpeuch.eu>
Co-committed-by: Antonin Delpeuch <antonin@delpeuch.eu>
(cherry picked from commit 2c185c39fe600041701d5f59cb1076a788815cb4)
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(cherry picked from commit b18c2e8d658c3311e0a299696bd1b6612c52ef13)
Conflicts:
services/migrations/gitlab.go
https://codeberg.org/forgejo/forgejo/pulls/2075
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