+ adjust scrolling workaround to obtain offsetTop from row parent instead of cell, now that individual cells have a top: 0

- now that cells are relative to the row parent, their offsetTop is 0

- Remove `@include euiDataGridRowCell` mixin and target the row directly

- Remove isStripableRow cell logic and instead use CSS nth-child for alternating stripes

- Swap stripes and highlight CSS so that highlight takes precendence over stripes without an !important

- Remove background color on individual cells to allow row colors to show through (NB: grid body is already set to the correct background color as well)

- skipping the mutation observer for now

- cells no longer have a set background color (to allow row color to bleed through), and we were previously relying on the grid body's bg color to be white

- using CSS `left`/`right` and `relative` on the inner grid parent solves the issue instead, as the row is now always correct width

- which does not work well due to children changing on virtualization

+ set additional visible row index data attribute for extra detail

+ use dataset

- was previously not working on prod, and now it does

This reverts commit aef8c64.