HParams: move selected checkbox behind header (#6476)

## Motivation for features / changes
The selection checkboxes display overtop of the selection header when
scrolling. This PR moves the DataTable's header row to a z-index of 1.
Which solves this problem and should solve any future issues like this
with the DataTable.

## Screenshots of UI changes (or N/A)
before:
<img width="48" alt="Screenshot 2023-06-30 at 10 18 34 AM"
src="https://github.com/tensorflow/tensorboard/assets/8672809/12c7e737-5b95-44fe-bde3-3c34e17b4676">

After:
<img width="55" alt="Screenshot 2023-06-30 at 10 18 12 AM"
src="https://github.com/tensorflow/tensorboard/assets/8672809/0181c2cc-6e3a-4d07-b458-c49b2378733b">

Author: JamesHollyer

tensorboard/webapp/widgets/data_table/data_table_component.scss

@@ -24,6 +24,7 @@ $_accent: map-get(mat.get-color-config($tb-theme), accent);
 
   .header {
     display: table-row;
+    z-index: 1;
   }
 
   .header {