Deprecation/removal of obsolete features

[cknitt]

This is an umbrella issue for obsolete features that should be deprecated/removed.

My suggestions would be to have them deprecated in an 11.1.x release and removed in v12.

• [x] JSX 3 (#6982)
• [ ] OCaml stdlib functions (#5779)
• [ ] bsconfig.json (#6781)
• [ ] -bs-g bsc-flag (#6496)
• [ ] js_cast.ml (#5439)

[cometkim]

6781 is waiting for v12.x release