Flymake diagnostic buffers have `next-error` support, but their
keymaps do not make use of it.  The attached patch by Matthew Tromp
makes `flymake-diagnostics-buffer-mode-map` and
`flymake-project-diagnostics-mode-map` more consistent with the other
`next-error` buffer keymaps, like those from occur, grep, and compile.

As discussed on emacs-devel:
https://mail.gnu.org/archive/html/emacs-devel/2025-05/msg00370.html