Summary

EUI 14.5.0 introduced some new semantics so I updated Kibana to match. Namely, EuiPageBody now is a main tag so I removed Kibana's overarching main and added main tag to individual pages that weren't using EuiPageBody.

While checking for landmarks on pages, I also fixed up nav landmark names because they're quite important.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

- [ ] This was checked for cross-browser compatibility, including a check against IE11

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
  - [ ] Documentation was added for features that require explanation or tutorials
  - [ ] Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately