Note that this PR cannot be cherrypicked into preview, as it relies on
the new settings infrastructure.

Switches settings from `show_scrollbars: "auto"` to `scrollbar: {show:
"auto", git_diffs: true}`.

fixes
https://linear.app/zed-industries/issue/Z-1650/scroll-bar-feature-settings

Release Notes:

- Changed scrollbar settings from `show_scrollbars: "auto"` to
`scrollbar: {show: "auto", git_diffs: true}`. (preview only)