Table: Fix remaining storybook a11y issues

`Table` has some a11y issues that should be fixed, particularly when working with subtables or footers. these are currently ignored from the storybook a11y tests (search for `a11y` in https://github.com/grafana/grafana/blob/main/packages/grafana-ui/src/components/Table/Table.story.tsx) you can see the problems locally by: - commenting out the rule ignores in the story - running storybook - going to a failing table story (e.g. `Footer`, `Pagination` or `Sub Tables`) - viewing the a11y test results - right hand sidebar, click addons -> accessibility once an individual problem is fixed, we can remove it entirely from the rule ignores in the story. once all problems are fixed, we can remove the entire a11y block! (it inherits from the root preview.ts) 🥳