Bug fix for updating to a prerelease module

[alerickson]

PR Summary

-Prerelease parameter was being ignored in Update-PSResource. This PR ensures both latestInstalledIsPrerelease and Prerelease are checked when determining whether to allow searching for a prerelease package.

PR Context

Resolves #1311

PR Checklist

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  • N/A or can only be tested interactively
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[ThomasNieto]

Please add a test using square bracket required version.