[APM] Add missing bottom border to header (elastic#93179)

Co-authored-by: Kibana Machine <[email protected]>

x-pack/plugins/apm/public/components/shared/ApmHeader/index.tsx

@@ -13,6 +13,7 @@ import { EnvironmentFilter } from '../EnvironmentFilter';
 const HeaderFlexGroup = euiStyled(EuiFlexGroup)`
   padding: ${({ theme }) => theme.eui.gutterTypes.gutterMedium};
   background: ${({ theme }) => theme.eui.euiColorEmptyShade};
+  border-bottom: ${({ theme }) => theme.eui.euiBorderThin};
 `;
 
 export function ApmHeader({ children }: { children: ReactNode }) {