Target FSharp.Core 6.0.0 as min supported version

[stephen-swensen]

• Target FSharp.Core 6.0.0 in main Unquote library project
• Create new UnquoteTests.FScore6 project that targets net8 and FSharp.Core 6.0.0 using external links to the main UnquoteTests project (which targets net8 and the latest FSharp.Core 8.0.403 giving us wide coverage)
• Update UnquoteTests and all Verify projects to target 8.0.403 (from 8.0.100)
• Update nuspec file (with FSharp.Core 6.0.0 dependency; also increment version number of unquote to 7.0.1)

Originated from this PR contributed by @bartelink: #170

resolves #169