Update to the latest TFM: net8.0

[oleksandr-didyk]

When source-building the product, only the latest versions of packages and their dependencies are used. With this in mind, it would be great for source-link to also target the latest TFM (net8.0) and not net7.0 as it does now

This would also allow the usage of latest runtime package dependencies, such as System.Text.JSON, which would otherwise require an undesirable warning suppression due to TFM miss-match

[MichaelSimons]

When doing this work, it is preferred that a new NetCurrent Arcade property be utilized as it eliminates this type of maintenance work for every release.

[mthalman]

Looks like @tmat is working on this: #936 (comment)

[MichaelSimons]

Looks like this can be closed now as fixed with #936.