chore!: Bump version to 8.0-dev for the Uno 7.0 line

Frees the 7.4-dev version space for servicing/7.4, which keeps publishing
packages for the previous Uno line. Without a bump both branches would compute
the same NBGV height and publish the same version from different code.

Taken as a major because the 7.0 line drops net9.0 -- every published asset is
currently net9 -- and removes the Mac Catalyst target framework, both breaking
changes by .NET convention regardless of API surface.

Author: agneszitte

version.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://raw.githubusercontent.com/dotnet/Nerdbank.GitVersioning/main/src/NerdBank.GitVersioning/version.schema.json",
-  "version": "7.4-dev.{height}",
+  "version": "8.0-dev.{height}",
   "nuGetPackageVersion": {
     "semVer": 2.0
   },