chore(react-reconciler/README.md): replace older Medium links with Software Mansion's 2025 Medium link

[onstash]

Reason: the older Medium links have not been updated with React 18+, hence out of sync.

Summary

How did you test this change?

[react-sizebot]

Comparing: 561ee24...0289bb2

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	+0.05%	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	606.22 kB	606.22 kB	=	107.38 kB	107.38 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	665.19 kB	665.19 kB	=	117.26 kB	117.26 kB
facebook-www/ReactDOM-prod.classic.js	=	689.06 kB	689.06 kB	=	121.30 kB	121.30 kB
facebook-www/ReactDOM-prod.modern.js	=	679.49 kB	679.49 kB	=	119.69 kB	119.69 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 0289bb2