Revert "EEVEE: Remove Thickness output"

This reverts commit 91b727028a.
The change was only meant for 4.1.
Thickness output is required for EEVEE-Next.

source/blender/nodes/shader/nodes/node_shader_output_material.cc

@@ -11,9 +11,7 @@ static void node_declare(NodeDeclarationBuilder &b)
   b.add_input<decl::Shader>("Surface");
   b.add_input<decl::Shader>("Volume").translation_context(BLT_I18NCONTEXT_ID_ID);
   b.add_input<decl::Vector>("Displacement").hide_value();
-  b.add_input<decl::Float>("Thickness")
-      .hide_value()
-      .unavailable() /* EEVEE-Next only. Does nothing in 4.1. */;
+  b.add_input<decl::Float>("Thickness").hide_value();
 }
 
 static int node_shader_gpu_output_material(GPUMaterial *mat,