Dashboard: Revert descending z-index changes

[leeoniya]

this depends on #83465, and should follow it.

reverts:

• Dashboard: Fix panels overlapping Save Drawer #76675
• Dashboard: Overflow and z-index adjustments #75597
• Dashboard: Set z-index of active panel to 999 #75382
• TooltipPlugin2: Raise parent grid item z-index when pinned #76379 (partial)
• Dashboard: Set descending z-index of panels in a dashboard (v2) #75328

it turned out to be too difficult to revert each PR in isolation due to conflicts. i did the first two successfully, but then employed a different strategy:

looking at the history of _dashboard_grid.scss, i restored it to the last commit before all of our z-index changes:

and then applied the only new changeset for reduced-motion:

---

still to be fixed:

• it looks like zIndex.activePanel has been used in recent work, so we need to figure out what to do in those two cases, and how to update them.

grafana/public/app/features/trails/DataTrail.tsx

Line 247 in 2540842

zIndex: theme.zIndex.activePanel + 1,

grafana/public/app/features/dashboard-scene/scene/DashboardControls.tsx

Line 77 in 2540842

zIndex: theme.zIndex.activePanel,

cc @torkelo ^^

[torkelo]

updated DataTrail and DasboardControls to use navbarFixed instead