Add "Upgrade NOTE:" as a Unity specific TODO pattern

When Unity automatically updates a shader due to being opened in a newer version, it will add notes to the top of the file, such as:

```
// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
```

`Upgrade NOTE:` should be added as a Unity specific TODO pattern that will highlight these comments and add them to the TODO explorer. A quick google search for `Unity "Upgrade NOTE"` shows results where these changes have caused issues. It would be good practise to treat these as TODO items to prompt testing and verifying the change.