add <PayloadOrganization> to MDM enrollment payload (#8752)

The value of this attribute is used by macOS in "System Preferences >
Profiles" to display a note saying "This Mac is supervised and managed
by <xyz>".

When <PayloadOrganization> is not set, it uses the URL of the MDM server
for the message, which looks unpolished.

server/service/apple_mdm.go

@@ -752,6 +752,8 @@ var enrollmentProfileMobileconfigTemplate = template.Must(template.New("").Parse
 	<string>{{ .Organization }} Enrollment</string>
 	<key>PayloadIdentifier</key>
 	<string>com.fleetdm.fleet.mdm.apple</string>
+	<key>PayloadOrganization</key>
+	<string>{{ .Organization }}</string>
 	<key>PayloadScope</key>
 	<string>System</string>
 	<key>PayloadType</key>