docs, build: replace the mailman url with the gitlab/issues url
Summary
Replace the old: http://lists.freedesktop.org/mailman/listinfo/polkit-devel URL, with the actual one: https://gitlab.freedesktop.org/polkit/polkit/-/issues/.
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Replace the old: http://lists.freedesktop.org/mailman/listinfo/polkit-devel URL, with the actual one: https://gitlab.freedesktop.org/polkit/polkit/-/issues/.