release: remove WORKSPACE snippets from release notes template (#4180)

## Summary

WORKSPACE support was removed in #4005 (released in 0.71.0). The release
notes template was not updated at the time, so every release since
0.71.0 (0.71.0, 0.71.1, 0.71.2, 0.71.3, 0.72.0) has advertised
`http_archive` WORKSPACE snippets that don't work.

This removes all WORKSPACE sections from the template so future release
notes only show the Bzlmod setup.

Also fixes a pre-existing missing `</details>` closing tag in the Prost
extension section.