Social Icons: update Medium icon

[megane9988]

What?

Update the Medium logo.

Why?

Medium's logo is changing: https://medium.design/logos-and-brand-guidelines-f1a01a733592
Our Social Icons logo should change too.

#57064

How?

This uses the new logo provided logo assets here : https://www.dropbox.com/s/fg1x3ekgvsn1h7q/Logo.zip?dl=0
https://www.dropbox.com/s/fg1x3ekgvsn1h7q/Logo.zip?dl=0

Testing Instructions

1. Go to Posts > Add New
2. Add a new Social Icons block
3. Add a new Medium block.

Testing Instructions for Keyboard

Screenshots or screencast

[t-hamano]

Sorry for the late review.

The icons are also centered exactly and the colors look correct 👍

Just to be sure, could you rebase this PR using the latest trunk? This is to run the test again and run the bot to collect Props.

[github-actions]

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Co-authored-by: megane9988 <megane9988@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: colinduwe <colind@git.wordpress.org>
Co-authored-by: andersnoren <anlino@git.wordpress.org>

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[t-hamano]

LGTM 👍