Update ERC-7683: Use uint40 for timestamps instead of uint32

[Zodomo]

Uint40 should be used for timestamps instead of uint32. In systems that use signed integers, an int32 value is only good until the year 2038. External systems might use signed alternatives because it allows them to utilize timestamps from before the Unix epoch. Uint40 would allow systems that use signed integers to be functional until the year ~19000. Such a simple change to enable these systems to be resilient for more than 14 years in this regard seems like a no-brainer.

[eip-review-bot]

File ERCS/erc-7683.md

Requires 1 more reviewers from @marktoda, @mrice32, @nicholaspai

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