content(blog): delete relaunching-pulumis-public-roadmap (#19541)

Remove the 2021 "Relaunching Pulumi's Public Roadmap" blog post and add an
S3 redirect from the old URL to /blog/ so the live URL and external links
don't 404.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: alexleventer

content/blog/relaunching-pulumis-public-roadmap/index.md

@@ -81,3 +81,4 @@ jobs/index.html|/careers/
 company/jobs/index.html|/careers/
 careers/jobs/index.html|/careers/
 company/pulumi/jobs/index.html|/careers/
+blog/relaunching-pulumis-public-roadmap/index.html|/blog/