Change OS update deadline to 7PM local time (#38810)

Updated documentation, backend, frontend, and tests to set the OS update
enforcement deadline to 19:00 (7PM) local time instead of noon. This
ensures consistency across user-facing text, API docs, configuration
files, and the MDM payload.

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #38834

# 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] Added/updated automated tests
- [x] QA'd all new/changed functionality manually

---------

Co-authored-by: Magnus Jensen <magnus@fleetdm.com>

ee/server/service/mdm.go

@@ -1307,7 +1307,7 @@ func (svc *Service) mdmAppleEditedAppleOSUpdates(ctx context.Context, teamID *ui
 	"Type": %q,
 	"Payload": {
 		"TargetOSVersion": %q,
-		"TargetLocalDateTime": "%sT12:00:00"
+		"TargetLocalDateTime": "%sT19:00:00"
 	}
 }`, softwareUpdateIdentifier, softwareUpdateType, updates.MinimumVersion.Value, updates.Deadline.Value))