Installcredprovider.ps1 should use .net 8 by default

[andyfisher100]

In https://github.com/microsoft/artifacts-credprovider/blob/master/helpers/installcredprovider.ps1As .net 6 is used as the default version of the credential provider to download.

.Net 6 is no longer a supported version and therefore I believe the default should be moved to .Net 8 but provide users who have not yet migrated their projects the option to specify the flag to use 6 if needed

[embetten]

This work is in progress; we are also going to be removing 3.1 and nefx 4.6. Since this is a breaking change for some folks, please see our announcements similar to #523.

[cobya]

This will be handled with #558 😄