rename ReactFiberContext to ReactFiberLegacyContext

[kassens]

It wasn't immediately obvious to me, that all the exports here are related to legacy context, so renaming for clarity.

Modern context lives in ReactFiberNewContext which we could probably also raname in a separate step to just Context.

[react-sizebot]

Comparing: fa3feba...d61733b

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.js	=	6.68 kB	6.68 kB	+0.11%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.57 kB	530.57 kB	=	93.67 kB	93.67 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	+0.05%	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	651.66 kB	651.66 kB	=	114.78 kB	114.78 kB
facebook-www/ReactDOM-prod.classic.js	=	674.81 kB	674.81 kB	=	118.78 kB	118.78 kB
facebook-www/ReactDOM-prod.modern.js	=	665.30 kB	665.30 kB	=	117.19 kB	117.19 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against d61733b