Fix pwsh dependant package version requirement to be exact version match.

As mentioned in Pwsh package discussion

Currently the update script creates a minimum version constraint on powershell-core, as opposed to an exact version requirement.
This means that in practice the all pwsh packages since 7.0.6+ and 7.1.3+ install the latest powershell-core, rather than the desired version of pwsh chosen.

This MR updates the script to set the dependant version to be an exact match e.g. [7.0.7] (instead of 7.0.7 which means >=7.0.7 ). Ref Nuget Version Ranges

Closes #2 (closed)