DOM attribute stringification fixes

This is regarding the discussion in #17773. React-DOM currently stringifies DOM attribute values before passing them to `Element.setAttribute(NS)` functions. This might be unnecessary, as these functions implicitly stringify attribute values on their own (WebIDL attributes typed as `DOMString`). It also makes it difficult to enforce [Trusted Types](https://w3c.github.io/webappsec-trusted-types/dist/spec/) in React applications, as the trusted type objects would be stringified before values reach the DOM sinks. Currently there is a `enableTrustedTypesIntegration` feature flag to disable stringification, but it seems like this behavior can be safely removed for modern browsers with no backwards-compatibility problems. Let me explain: Attribute stringification was introduced in https://github.com/facebook/react/commit/b0455f46709fca94da0b6126b719d6dd07605e65, at that time to workaround a [**jsdom**](https://github.com/jsdom/jsdom) limitation (jsdom's DOM emulation didn't stringify on its own). **IE 8/9** [have a similar issue](https://github.com/facebook/react/commit/3b94abcb3b2c05fc38406b02a501427e58377c44). If an object is passed to a DOM attribute, its value becomes `[object]`, ignoring any stringification rules defined in objects' `toString` function. - Jsdom does not have the issue anymore. Since at least 4.0.0 its `setAttribute` function does stringify the values via its IDL layer ([runkit demo](https://runkit.com/kkotowicz/jsdom-setattribute)). - React doesn't support IE 8 anymore. - The issue still exists for IE9 (contrary to https://github.com/facebook/react/issues/11735, my tests confirm that the bug still exists, but one needs to try a standard attribute, like `p.title`, and not one with a custom name). - All other browsers, even in their old versions (I tested IEs, Firefox, Chrome, Safari, Opera and a few mobile browsers ) correctly stringify. I propose to remove the stringification (similar to https://github.com/facebook/react/pull/17774) unless a browse…