Fix scrollbar in upgrade modal (#7076)

## Issue
5848 Remove scroll-bar in update prompt

## Changes
- Make scrollbars appear only when necessary.
- Style the scrollbar. Similar to other areas, we skip this for Mac (can't recall why we need to skip).

ui/modal/modalUpgrade/view.jsx

@@ -3,6 +3,8 @@ import React from 'react';
 import { Modal } from 'modal/modal';
 import LastReleaseChanges from 'component/lastReleaseChanges';
 
+const IS_MAC = navigator.userAgent.indexOf('Mac OS X') !== -1;
+
 type Props = {
   downloadUpgrade: () => void,
   skipUpgrade: () => void,
@@ -14,6 +16,7 @@ class ModalUpgrade extends React.PureComponent<Props> {
 
     return (
       <Modal
+        className={IS_MAC ? '' : 'main-wrapper--scrollbar'}
         isOpen
         contentLabel={__('Upgrade available')}
         title={__('LBRY leveled up')}

ui/scss/init/_gui.scss

@@ -485,7 +485,7 @@ textarea {
 
 .release__notes {
   max-height: 50vh;
-  overflow: scroll;
+  overflow: auto;
 }
 
 .signup__odysee-logo {

web/scss/themes/odysee/init/_gui.scss

@@ -488,7 +488,7 @@ textarea {
 
 .release__notes {
   max-height: 50vh;
-  overflow: scroll;
+  overflow: auto;
 }
 
 .signup__odysee-logo {