Update Medium logo for Social Link Block

[colinduwe]

What?

Updates the Medium logo for the Social Link Block

Why?

Issue: #57064
Medium updated its logo in 2020.

How?

Change the svg in two places. Note that I kept the spacing noted in Medium's brand guidelines. I'm inclined to remove at least some of that space now that I see it it place beside other logos.

Testing Instructions

1. Add a Social Links block and add at least the Medium social link
2. Notice that it has the correct "three elipse" symbol instead of the capital "M" logo.

Screenshots or screencast

[github-actions]

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[t-hamano]

@colinduwe Thanks for the PR!

I found out that #57073 has already been submitted before this PR. Sorry, let me close this PR and let #57073 move forward.

[github-actions]

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