mirror-pool-dispatcher: bump maximum expected mirrors.json size to 32 KiB (refs: #12405)

This is supposed to have been done in
commit:94f7a231 already
but apparently the merge commit:c5eeb2b4
dropped this change for some reason.

mirror-pool-dispatcher @ 99bb6898

-Subproject commit 640e250df14d11bf1b158f37b4dda6a49363c428
+Subproject commit 99bb6898d04f57995835044520f6192c50b62399