Update javascript-backend to js-backend and javascript-frontend to js-frontend

[NWylynko]

🔎 Previews:

• https://clerk-git-nick-sdk-key-migration.clerkstage.dev/docs/pr/2096/quickstarts/javascript
• https://clerk-git-nick-sdk-key-migration.clerkstage.dev/docs/pr/2096/components/authentication/sign-in
• https://clerk-git-nick-sdk-key-migration.clerkstage.dev/docs/pr/2096/references/backend/overview

What does this solve?

• We want to change javascript-backend to js-backend and javascript-frontend to js-frontend

What changed?

• updated all instances

Checklist

• I have clicked on "Files changed" and performed a thorough self-review
• I have added the "deploy-preview" label and added the preview link(s) to this PR description
• All existing checks pass

This is blocked on the merge of https://github.com/clerk/clerk/pull/1065

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/2096