Document eth_sign discontinuation

[alexandratran]

Description

Update eth_sign description to note that it is no longer supported.

Issue(s) fixed

Fixes #1313

Preview

https://docs.metamask.io/1313-eth-sign-discontinuation/wallet/concepts/signing-methods/#eth_sign

Checklist

Complete this checklist before merging your PR:

• [x] If this PR contains a major change to the documentation content, I have added an entry to the top of the "What's new?" page.
• [x] The proposed changes have been reviewed and approved by a member of the documentation team.

[github-actions[bot]]

Preview published: 1313-eth-sign-discontinuation

[vandan]

Only thought is to try and add a reference to the associated MIP for this change. It's still an outstanding PR that needs to be reviewed/merged - should happen very soon: https://github.com/MetaMask/metamask-improvement-proposals/pull/42

[github-actions[bot]]

Preview published: 1313-eth-sign-discontinuation

[alexandratran]

Added in a link for MIP-3, but the file is not ready due to autonumbering issues: https://github.com/MetaMask/metamask-improvement-proposals/blob/main/MIPs/mip-x.md.

Once that file is updated to mip-3.md, this PR should be ready to go.

[github-actions[bot]]

Preview published: 1313-eth-sign-discontinuation