[PacMan] Update option not available when package manager UI downgraded to lower version

1. What happened
Update option not available when package manager UI downgraded to lower version

2. How we can reproduce it using the example you attached
1. Install Unity.
2. Create a new project.
3. Click Window> Package Manager
4. Go to Package Manager UI under "All" tab.
5. Click "Go back to 1.8.2".

Observe: No option available to update it back to latest version

Expected Result : User should get an option to update it back to latest version.

Notes:
- Appears always
- Appears on both Mac and Windows
- PackMan UI latest version available on 2018.1.0f1 is 1.8.7 and on 2018.2.0b2 is 1.9.3

Work Around:
- Go to Help > Reset Packages to Default

Occuring On: 2018.1.0f1 , 2018.2.0b1
Works fine On: 2018.1.0b13 , 2018.1.0b6