feat(repo): remove node-fetch dependency, require Node.js 20+

[mandarini]

Remove @supabase/node-fetch polyfill in favor of native fetch API available in Node.js 20+. This change drops support for Node.js 18, which reached end-of-life on April 30, 2025.

All target environments (Node 20+, browsers, Deno, Bun, React Native) have native fetch support. No API changes for users on supported platforms.

Changes:

• Remove @supabase/node-fetch dependency from all packages
• Simplify fetch resolution logic to use native fetch
• Add Node.js >= 20 engine constraint to all packages
• Update documentation with migration guide
• Remove Node 18 CI workflow

See: https://github.com/orgs/supabase/discussions/37217

Node.js 18 is no longer supported. Minimum version is now Node.js 20.

Breaking

This will be a breaking change since we're not supporting Node 18 any more.

[coveralls]

coverage: 80.698% (-0.6%) from 81.278%
when pulling dafa160 on feat/remove-node-fetch
into 7511686 on master.

[soedirgo]

Some failures in auth-js tests