Unify DEP spelling with a space instead of dash (e.g. "DEP 0")

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

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

Arguments in favor of using a space include being aligned with how e.g. Python (https://peps.python.org/) and RFCs do it (https://datatracker.ietf.org/doc/html/rfc2822).

Note all three alternatives:

• !13 (merged): DEP-0
• !14 (closed): DEP 0
• !15 (closed): DEP0

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