(#890) Don't display splash screen on top

[garas]

Description Of Changes

Remove second parameter of SplashScreen.Show(bool autoClose, bool topMost).

https://learn.microsoft.com/en-us/dotnet/api/system.windows.splashscreen.show

Motivation and Context

Splash screen displayed on top while Chocolatey GUI starts up, unable use computer during this time.

Testing

1. Open web browser (or any other application), maximize its window.
2. Start Chocolatey GUI.
3. While splash screen is displayed, activate the web browser window.
4. Chocolatey GUI splash screen should go behind web browser window.

Change Types Made

• Bug fix (non-breaking change).
• Feature / Enhancement (non-breaking change).
• Breaking change (fix or feature that could cause existing functionality to change).
• Documentation changes.
• PowerShell code changes.

Change Checklist

• Requires a change to the documentation.
• Documentation has been updated.
• Tests to cover my changes, have been added.
• All new and existing tests passed?
• PowerShell code changes: PowerShell v2 compatibility checked?

Related Issue

Fixes #890