fix inconsistent header spacing by using section header (#33095)

fixes: #30166 

Opted to use the already existing `SectionHeader` component, and it's
subtitle prop, that way we stay consistent across all pages in the
entire product.

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

## Testing

- [x] QA'd all new/changed functionality manually

## Media:

<img width="538" height="170" alt="image"
src="https://github.com/user-attachments/assets/d54b6b1c-d864-43ab-ac3b-5308267d4610"
/>

<img width="685" height="116" alt="image"
src="https://github.com/user-attachments/assets/872bd916-59cf-4e50-b1c8-6e3647008fc0"
/>

changes/30166-inconsistent-spacing-os-settings-cards-headers

@@ -0,0 +1 @@
+* Fixed inconsistent spacing in Controls OS settings headers