Replace all occurrences of `DEP ([0-9]{1,2})` with `DEP-N` to ensure
spelling is consistent with the `DEP-0` spelling in the title of the
original `DEP-0` proposal.

Likewise, replace all occurrences of `DEP([0-9]{1,2})` without a space
with `DEP-N` with a dash.

Do not change filenames, but allow them to stay as `depN` or whatever.