Add `upgraded` field to EIP-template.

[xinbenlv]

trafficstars

I create a demonstrative change to eip-template.md for adding upgrade.

This was based on a suggestion from EIPIP meeting today, in which @lightclient @SamWilsn participated

Could you see if this describe a good understanding of the need from Core EIPs? Once we reach a good form, I will update accordingly EIP-1 and elsewhere.

[eth-bot]

Hi! I'm a bot, and I wanted to automerge your PR, but couldn't because of the following issue(s):

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(fail) eip-template.md

classification
ambiguous

• 'eip-template.md' must be in eip-###.md format; this error will be overwritten upon relevant editor approval