Update ERC-7913: Move to Final

[Amxx]

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[eip-review-bot]

File ERCS/erc-7913.md

Requires 1 more reviewers from @g11tech, @SamWilsn, @xinbenlv

[SamWilsn]

In the last line of the motivation, you say "the key is empty", but on the first line of the nomenclature section, you say "keys MUST NOT be empty". Is this not a contradiction?