Remove legacy ReactDOM APIs behind feature flag

[kassens]

These APIs have been deprecated with a warning since React 17. This disables them behind a feature flag for a major release.

[react-sizebot]

Comparing: 8039e6d0b93b642a4bd1ee7dbe108d2153fd2981...cf0cfea13cda6bbee2ac10a9df946b1ffcebddb9

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	=	175.09 kB	173.72 kB	=	54.47 kB	54.16 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	177.21 kB	175.84 kB	=	55.16 kB	54.85 kB
facebook-www/ReactDOM-prod.classic.js	=	567.70 kB	567.74 kB	=	99.96 kB	99.96 kB
facebook-www/ReactDOM-prod.modern.js	=	551.56 kB	551.56 kB	=	97.05 kB	97.05 kB

Significant size changes

Includes any change greater than 0.2%:

Expand to show

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-experimental/react-dom/umd/react-dom.profiling.min.js	=	185.99 kB	184.64 kB	=	57.83 kB	57.45 kB
oss-experimental/react-dom/cjs/react-dom.profiling.min.js	=	186.89 kB	185.52 kB	=	57.60 kB	57.31 kB
oss-stable/react-dom/umd/react-dom.profiling.min.js	=	183.88 kB	182.53 kB	=	57.15 kB	56.80 kB
oss-stable-semver/react-dom/umd/react-dom.profiling.min.js	=	183.80 kB	182.45 kB	=	57.12 kB	56.78 kB
oss-stable/react-dom/cjs/react-dom.profiling.min.js	=	184.76 kB	183.39 kB	=	56.94 kB	56.64 kB
oss-stable-semver/react-dom/cjs/react-dom.profiling.min.js	=	184.68 kB	183.32 kB	=	56.91 kB	56.61 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.production.min.js	=	183.42 kB	182.05 kB	=	57.53 kB	57.22 kB
oss-experimental/react-dom/cjs/react-dom-unstable_testing.development.js	=	1,360.39 kB	1,350.12 kB	=	300.53 kB	298.72 kB
oss-experimental/react-dom/umd/react-dom.development.js	=	1,406.91 kB	1,396.20 kB	=	299.20 kB	297.36 kB
oss-experimental/react-dom/umd/react-dom.production.min.js	=	176.96 kB	175.61 kB	=	55.39 kB	55.02 kB
oss-experimental/react-dom/cjs/react-dom.development.js	=	1,342.34 kB	1,332.07 kB	=	296.17 kB	294.35 kB
oss-stable/react-dom/umd/react-dom.development.js	=	1,393.12 kB	1,382.41 kB	=	296.71 kB	294.87 kB
oss-stable-semver/react-dom/umd/react-dom.development.js	=	1,393.09 kB	1,382.38 kB	=	296.68 kB	294.85 kB
oss-stable/react-dom/cjs/react-dom.development.js	=	1,329.22 kB	1,318.95 kB	=	293.74 kB	291.92 kB
oss-stable-semver/react-dom/cjs/react-dom.development.js	=	1,329.19 kB	1,318.92 kB	=	293.71 kB	291.89 kB
oss-stable/react-dom/umd/react-dom.production.min.js	=	174.85 kB	173.50 kB	=	54.76 kB	54.42 kB
oss-stable-semver/react-dom/umd/react-dom.production.min.js	=	174.77 kB	173.42 kB	=	54.73 kB	54.40 kB
oss-experimental/react-dom/cjs/react-dom.production.min.js	=	177.21 kB	175.84 kB	=	55.16 kB	54.85 kB
oss-stable/react-dom/cjs/react-dom.production.min.js	=	175.09 kB	173.72 kB	=	54.47 kB	54.16 kB
oss-stable-semver/react-dom/cjs/react-dom.production.min.js	=	175.01 kB	173.64 kB	=	54.45 kB	54.13 kB

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