Disable strict effects for legacy roots (again)

[bvaughn]

See #21590 (comment) for context.

During the splitting of <StrictMode> into separate levels (#20849) the initial decision to disable strict effects for legacy roots was lost. This PR updates the mode assignment to be based on root mode as before. As of this PR, there are only two places we enable strict effects mode:

createHostRootFiber() if a strict flag is specified:

export function createHostRootFiber(
  tag: RootTag,
  isStrictMode: boolean,
  concurrentUpdatesByDefaultOverride: null | boolean
): Fiber {
  let mode;
  if (tag === ConcurrentRoot) {
    mode = ConcurrentMode;
    if (isStrictMode === true) {
      mode |= StrictLegacyMode;

      if (enableStrictEffects) {
        mode |= StrictEffectsMode;
      }
    } else if (enableStrictEffects && createRootStrictEffectsByDefault) {
      mode |= StrictLegacyMode | StrictEffectsMode;
    }

createFiberFromTypeAndProps() if <StrictMode> is used within a modern root:

mode |= StrictLegacyMode;
if (enableStrictEffects && (mode & ConcurrentMode) !== NoMode) {
  // Strict effects should never run on legacy roots
  mode |= StrictEffectsMode;
}

[sizebot]

Comparing: d75105f...6efce75

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.min.js	=	126.00 kB	126.00 kB	=	40.41 kB	40.41 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	128.82 kB	128.82 kB	=	41.35 kB	41.35 kB
facebook-www/ReactDOM-prod.classic.js	+0.02%	406.09 kB	406.15 kB	+0.02%	75.11 kB	75.12 kB
facebook-www/ReactDOM-prod.modern.js	+0.02%	394.46 kB	394.52 kB	+0.02%	73.29 kB	73.30 kB
facebook-www/ReactDOMForked-prod.classic.js	+0.02%	406.09 kB	406.15 kB	+0.02%	75.11 kB	75.12 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 6efce75