Refactored version directory names

3.0 and 2.0 do not work, there are multiple versions and anything with a period in it sucks from my point of view, v2 and v3 feel much better from a DX perspective.

datamodel/low/v3/server_variable.go

@@ -0,0 +1,11 @@
+package v3
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+)
+
+type ServerVariable struct {
+	Enum        []low.NodeReference[string]
+	Default     low.NodeReference[string]
+	Description low.NodeReference[string]
+}