Deprecate legacy context within StrictMode tree

[bvaughn]

Removed warnAboutLegacyContextAPI feature flag (and always warn about legacy context inside of StrictMode tree). Updated tests with legacy context to expect warnings.

Note that I will be on PTO from Oct 4th - Oct 18th and will be unresponsive to PR feedback. All React core team members have been added as collaborators to my fork. Please feel free to make changes directly to this branch before merging.

[sizebot]

Details of bundled changes.

Comparing: 3c60f32...2f1ab4c

react

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react.development.js	-0.0%	-0.1%	91.68 KB	91.65 KB	24.27 KB	24.26 KB	UMD_DEV
react.development.js	-0.1%	-0.1%	56.02 KB	55.99 KB	15.33 KB	15.32 KB	NODE_DEV
React-dev.js	-0.7%	-1.0%	52.48 KB	52.13 KB	14.45 KB	14.3 KB	FB_WWW_DEV

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom.development.js	-0.0%	-0.0%	658.84 KB	658.72 KB	153.9 KB	153.87 KB	UMD_DEV
react-dom.development.js	-0.0%	-0.0%	654.17 KB	654.06 KB	152.52 KB	152.48 KB	NODE_DEV
react-dom-server.browser.development.js	-0.0%	-0.1%	106.12 KB	106.09 KB	28.22 KB	28.2 KB	UMD_DEV
react-dom-server.browser.development.js	-0.0%	-0.1%	102.25 KB	102.22 KB	27.22 KB	27.21 KB	NODE_DEV
react-dom-server.node.development.js	-0.0%	-0.0%	104.18 KB	104.14 KB	27.78 KB	27.77 KB	NODE_DEV
ReactDOM-dev.js	-0.1%	-0.1%	671.44 KB	671 KB	153.24 KB	153.09 KB	FB_WWW_DEV
ReactDOMServer-dev.js	-0.3%	-0.6%	102.06 KB	101.71 KB	26.64 KB	26.48 KB	FB_WWW_DEV

react-art

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-art.development.js	-0.0%	-0.0%	448.78 KB	448.67 KB	100.32 KB	100.28 KB	UMD_DEV
react-art.development.js	-0.0%	-0.0%	380.56 KB	380.44 KB	83.15 KB	83.11 KB	NODE_DEV
ReactART-dev.js	-0.1%	-0.2%	384.59 KB	384.15 KB	81.68 KB	81.53 KB	FB_WWW_DEV

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
scheduler.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD
scheduler-tracing.development.js	-0.3%	-0.7%	10.16 KB	10.12 KB	2.31 KB	2.29 KB	NODE_DEV
SchedulerTracing-dev.js	-3.4%	-7.7%	10.27 KB	9.91 KB	2.25 KB	2.07 KB	FB_WWW_DEV

Generated by 🚫 dangerJS

[bvaughn]

Got approval from Sebastian via a Messenger message so I'm going to land this one~