React UI: Make link to classic UI work (#6320)

It being a Reach Router <Link> caused the Reach router to not actually
leave the React app, even though the destination path was not a path
handled by the Reach Router.

Signed-off-by: Julius Volz <julius.volz@gmail.com>

web/ui/react-app/src/Navbar.tsx

@@ -67,9 +67,7 @@ const Navigation: FC<PathPrefixProps> = ({ pathPrefix }) => {
             <NavLink href="https://prometheus.io/docs/prometheus/latest/getting_started/">Help</NavLink>
           </NavItem>
           <NavItem>
-            <NavLink tag={Link} to={pathPrefix}>
+            <NavLink href={`${pathPrefix}/`}>Classic UI</NavLink>
-              Classic UI
-            </NavLink>
           </NavItem>
         </Nav>
       </Collapse>