docs: list browser versions that already have native fetch

[Hashim1999164]

Summary

• Expands the Browser Support section to call out when whatwg-fetch is unnecessary.
• Adds a table of minimum browser versions that already ship native window.fetch.
• Keeps the existing note that native implementations should be reported to browser vendors, not this polyfill.

Fixes #1483

Test plan

• Read the updated Browser Support section in the README preview
• Confirm the version table matches common caniuse / MDN guidance for native fetch