pcb: update stackup, golden finger specs & SE75 ref layer

- Update stackup materials to use TU-872SLK
- Update trace width and spacing for impedance control traces
- F.Cu SE75 now references to L3.Cu GND
- Clarify golden fingers dimensions

src/cxp_sfp.kicad_pro

@@ -504,9 +504,9 @@
         "wire_width": 6
       },
       {
-        "clearance": 0.0,
-        "diff_pair_gap": 0.3,
-        "diff_pair_width": 0.28,
+        "clearance": 0.2,
+        "diff_pair_gap": 0.2,
+        "diff_pair_width": 0.3255,
         "name": "100R",
         "pcb_color": "rgba(0, 0, 0, 0.000)",
         "priority": 1,
@@ -521,7 +521,7 @@
         "pcb_color": "rgba(0, 0, 0, 0.000)",
         "priority": 0,
         "schematic_color": "rgba(0, 0, 0, 0.000)",
-        "track_width": 0.153
+        "track_width": 0.2144
       }
     ],
     "meta": {