Update net7.0 TFM references to net8.0

[Michael Simons]

In order to be source-build compliant for the .NET 8.0 release. All projects targeting a net7.0 TFM must be upgraded to net8.0. It is preferred that a new NetCurrent Arcade property be utilized as it eliminates this type of maintenance work for every release.

[Matt Thalman]

@jmarolf - Can you take a look at this? It looks like there hasn't been a recent merge of Arcade (#1795) which would be needed to enable this.