[WEB-4012] Update neutral color ramp in Ably UI

Update the neutral color ramp in the CSS variables.

* Update the CSS variables for the new neutral shades in `src/core/styles/properties.css`
* Remove the old neutral shades from the CSS variables in `src/core/styles/properties.css`

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src/core/styles/properties.css

@@ -7,12 +7,12 @@
     --color-neutral-300: #e6eaf0;
     --color-neutral-400: #e2e7ef;
     --color-neutral-500: #c6ced9;
-    --color-neutral-600: #adb6c2;
-    --color-neutral-700: #89929f;
-    --color-neutral-800: #667085;
-    --color-neutral-900: #39414e;
-    --color-neutral-1000: #2b303b;
-    --color-neutral-1100: #202531;
+    --color-neutral-600: #a7b1be;
+    --color-neutral-700: #8992a4;
+    --color-neutral-800: #687288;
+    --color-neutral-900: #5b5a72;
+    --color-neutral-1000: #434356;
+    --color-neutral-1100: #2c3344;
     --color-neutral-1200: #141924;
     --color-neutral-1300: #03020d;