Addon-docs: Remove mdx1-csf as optional peer dep

[shilman]

Closes #21935

What I did

Remove mdx1-csf as optional peer dep. This will break strict package manager support for MDX1 (yarn pnp, pnpm) but will eliminate the security warnings for npm users, who are our largest user population by far).

How to test

In a sandbox:

• Install @storybook/mdx1-csf as a dev dependency
• Set features.legacyMdx1 = true in main.js
• Run the Storybook
• Observe that the an MDX2 expression (e.g. {1 + 1} inlined) is not evaluated

[socket-security]

New dependency changes detected. Learn more about Socket for GitHub ↗︎

---

👍 No new dependency issues detected in pull request

Bot Commands

To ignore an alert, reply with a comment starting with @SocketSecurity ignore followed by a space separated list of package-name@version specifiers. e.g. @SocketSecurity ignore foo@1.0.0 bar@* or ignore all packages with @SocketSecurity ignore-all

Pull request alert summary

Issue	Status
Install scripts	✅ 0 issues
Native code	✅ 0 issues
Bin script confusion	✅ 0 issues
Bin script shell injection	✅ 0 issues
Shell access	✅ 0 issues
Uses eval	✅ 0 issues
Unresolved require	✅ 0 issues
Invalid package.json	✅ 0 issues
HTTP dependency	✅ 0 issues
Git dependency	✅ 0 issues
GitHub dependency	✅ 0 issues
New author	✅ 0 issues
Potential typo squat	✅ 0 issues
Known Malware	✅ 0 issues
Telemetry	✅ 0 issues
Protestware/Troll package	✅ 0 issues

---

📊 Modified Dependency Overview:

🚮 Removed packages: @babel/core@7.12.9, @storybook/mdx1-csf@1.0.0

[shilman]

Self-merging @ndelangen