[PM-5052] Upgrade to .NET 8 (#3461)

* Upgrade to .NET 8

* Linting

* Clean up old JSON deserialization code

* More .NET 8-oriented linting

* Light feedback

* Get rid of old test we don't know the root issue for

* Fix a new test

* Remove now-unnecessary Renovate constraint

* Use Any()

* Somehow a 6.0 tooling config we don't need snuck back in

* Space out properties that always change per release

* Bump a few core packages since the last update

src/Core/Utilities/SpanExtensions.cs

@@ -18,21 +18,4 @@ public static class SpanExtensions
         rest = input[++splitIndex..];
         return true;
     }
-
-    // Replace with the implementation from .NET 8 when we upgrade
-    // Ref: https://github.com/dotnet/runtime/issues/59466
-    public static int Count<T>(this ReadOnlySpan<T> span, T value)
-        where T : IEquatable<T>
-    {
-        var count = 0;
-        int pos;
-
-        while ((pos = span.IndexOf(value)) >= 0)
-        {
-            span = span[++pos..];
-            count++;
-        }
-
-        return count;
-    }
 }