fix(theme): forward className prop in theme-classic's Heading

[JoshuaKGoldberg]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• [ ] If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

See #8327.

Apologies for taking a good first issue after I've already sent a couple PRs to this repo. I just was excited about this one 😄

Test Plan

I tried this out locally on the typescript-eslint/typescript-eslint#5951 PR and it worked.

Test links

I don't see any existing tests for this. What would you like me to do?

Related issues/PRs

Fixes #8327.

[netlify]

✅ [V2]

Built without sensitive environment variables

Name	Link
🔨 Latest commit	f2aedca
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/63750e8129908200092cfbf9
😎 Deploy Preview	https://deploy-preview-8350--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 85	🟢 97	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 75	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

:D

Apparently this is the only thing that should be done.

The TS side already existed and we just did not merge classes properly

  export interface Props extends ComponentProps<HeadingType> {
    readonly as: HeadingType;
  }

We don't have good tests on the UI, will add a storybook later but it's ok for now 👍