Rename update feed deployment environment

[mstallone]

TL;DR

Rename the GitHub deployment environment shown on the repository homepage from github-pages to Update Feed.

What was happening

• The backing branch, workflow, and job used update-feed terminology.
• GitHub's homepage still displayed the underlying github-pages environment name under Deployments.

What this changes

• Deploy future Pages artifacts through the Update Feed environment.
• Update the release setup documentation to match the visible environment name.
• Preserve the existing main-only deployment branch policy on the renamed environment.

Heads-up

• GitHub Pages remains the hosting transport and the public URLs do not change.
• The old environment will be retired only after a successful deployment through Update Feed.

Tests

• Workflow YAML parsing passed.
• Whitespace validation passed.