upgrade stories & events grid to grid2

[ChristopherChudzicki]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/6007

Description (What does it do?)

This PR fixes a bug that was introduced when MUI was updated from v5 to v6 as part of #1776.

MUI v5 had Grid and Unstable_Grid2 components. In v6, Grid2 was stabilized Grid was deprecated. Additionally, v6 seems to have introduced a bug in Grid. The bug only affects Grid usage that has string-values for columnSpacing. We had exactly one instance of string-valued columnSpacing. So I've upgraded that particular usage to Grid2 and everything looks fine now.

See mui/material-ui#44321 for MUI bug.

How can this be tested?

1. On this branch, compare styles of Stories & Events section of the homepage with that on https://learn.mit.edu/. (The bug has not yet made it to production). When comparing styles, I recommend:
   • Put local branch in browser Tab 1, production in Tab 2
   • Ensure zoom level is same in both tabs
   • Ensure both tabs have same widths (e.g., close dev tools on both tabs)
   • swap between Tab 1 and Tab 2.
2. Suggest doing additional style comparison of other pages, e.g. /departments, /topics, /units, /search, /dashboard + its tabs, and channel pages.
   • when comparing, ensure both tabs have scrollbar or both tabs do not have scrollbar. E.g., if viewing /dashboard/my-lists, if one tab has 1 list and the other has 10 lists, scrollbar might affect positioning in the longer list.

[jonkafton]

Looks good / fixed. Instances of Grid v1 also good.