[docs] Update GH action project ID

Update the Vercel project iD for legacy docs GH action.

## Summary & Motivation

## How I Tested These Changes

## Changelog

> Insert changelog entry or delete this section.

Author: hellendag

.github/workflows/build-docs.yml

@@ -71,7 +71,7 @@ jobs:
           github-comment: ${{ github.event.pull_request && env.CHANGES_ENTRY || true }}
           vercel-token: ${{ secrets.VERCEL_TOKEN }}
           vercel-org-id: ${{ secrets.VERCEL_ORG_ID }}
-          vercel-project-id: ${{ secrets.VERCEL_PROJECT_ID }}
+          vercel-project-id: ${{ secrets.LEGACY_DOCS_VERCEL_PROJECT_ID }}
           github-token: ${{ secrets.GITHUB_TOKEN }}
           scope: ${{ secrets.VERCEL_ORG_ID }}
           alias-domains: ${{ env.BRANCH_PREVIEW_SUBDOMAIN }}.dagster.dagster-docs.io