docs: remove outdated credentials omit warning

[irekrog]

Summary

• Remove the outdated credentials: 'omit' limitation from the current Networking documentation.
• Apply the same correction to the React Native 0.86 documentation.

React Native 0.86.0 was verified on an iOS 26.5 simulator and an Android API 37 emulator. All 10 credential and cookie scenarios described in the linked issue passed independently on both platforms, including direct requests and automatic redirects.

Test plan

• yarn lint:website
• yarn prettier --check docs/network.md website/versioned_docs/version-0.86/network.md
• yarn --cwd website lint:markdown:links

Closes react/react-native#57786

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