[Flight] Fix markup types for server references

[eps1lon]

Follow-up to d5324f4

Flow narrows the ServerReferenceId to a string due to typeof check and then it can't assign the narrowed value to an opaque type.

Instead we don't make ServerReferenceId opaque like for the other configs.

[react-sizebot]

Comparing: 1068027...4f4b182

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	=	6.84 kB	6.84 kB	=	1.88 kB	1.88 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	608.67 kB	608.67 kB	=	107.63 kB	107.63 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.84 kB	6.84 kB	+0.05%	1.88 kB	1.88 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	674.60 kB	674.60 kB	=	118.58 kB	118.58 kB
facebook-www/ReactDOM-prod.classic.js	=	693.92 kB	693.92 kB	=	121.98 kB	121.98 kB
facebook-www/ReactDOM-prod.modern.js	=	684.31 kB	684.31 kB	=	120.37 kB	120.37 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

Generated by 🚫 dangerJS against 4f4b182