docs: remove pulse.cio.gov mentions from README

[flotwig]

🗣 Description

The pulse repository was archived on GitHub in May 2020. Additionally, the website at https://pulse.cio.gov no longer loads. This mention should be removed.

💭 Motivation and context

See description

🧪 Testing

n/a

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All future TODOs are captured in issues, which are referenced in code comments.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] Tests have been added and/or modified to cover the changes in this PR.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [x] Revert dependencies to default branches.
• [x] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.