Allow Update-PSModuleManifest to remove pre-release tag

Summary of the new feature / enhancement

I am a module author. Until recently, I have been using the BuildHeplers module from GitHub user RamblingCookieMonster. One thing that module has is the ability to update the module manifest, though, it has limitations.

So I was perusing the code of this project and came across the UpdateModuleManifest.cs class. And lo and behold, it's a cmdlet! Awesome, I can remove my projects' dependency on BuildHelpers, which is all but abandonware at this point.

But then I came across these lines of code in UpdateModuleManifest.cs:

https://github.com/PowerShell/PSResourceGet/blob/e71e500d3aa964d39fb29598a1f3427ccabb75c1/src/code/UpdateModuleManifest.cs#L233-L234

So now I'm left wondering, as a module author, how would I go about, in an automated fashion—during a build, for example—removing a pre-release tag when my module goes GA?

Proposed technical implementation details (optional)

Since this cmdlet is meant to update a module's manifest, I would expect that I could execute the following to effectively move to a GA status, using this project as an example:

Update-PSModuleManifest -Path './src/MIcrosoft.PowerShell.PSResourceGet.psd1' -Version 0.5.25 -PreRelease ''

This bumps the version of the module to the next patch version and removes the pre-release tag in the metadata section of the module manifest.

So the proposed change is the removal of the [ValidateNotNullOrEmpty] attribute from this parameter property of the cmdlet.

Start in src/code/UpdateModuleManifest.cs at the PreRelease parameter and the referenced ValidateNotNullOrEmpty attribute. Update-PSModuleManifest should accept an empty -PreRelease value and remove the pre-release tag from the manifest when invoked with the example command.