Mention Workbench in RN w.r.t. #5612

[rstub]

Intent

Adds mention of Workbench in the release notes w.r.t. rstudio-pro#5612.

Approach

Minimal changes adding workbench" in the "RStudio" section. Alternative would be a new bbullet point in the "Workbench" section.

Automated Tests

NA

QA Notes

NA

Documentation

Update release notes.

Checklist

• If this PR adds a new feature, or fixes a bug in a previously released version, it includes an entry in NEWS.md
• If this PR adds or changes UI, the updated UI meets accessibility standards
• A reviewer is assigned to this PR (if unsure who to assign, check Area Owners list)
• This PR passes all local unit tests

[gtritchie]

LGTM!