fix(learn): update guide for v20

[avivkeller]

Fixes #7914

cc @mcollina

[vercel]

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Project	Deployment	Preview	Updated (UTC)
nodejs-org	Ready	Preview	Sep 1, 2025 1:14pm

[Copilot]

Pull Request Overview

This PR updates the event loop documentation to clarify behavior changes introduced in Node.js v20. The change adds important information about libuv 1.45.0's modified timer execution behavior in the event loop.

Key Changes

• Adds clarification about event loop timing changes in Node.js v20
• Documents the shift in timer execution from before/after poll phase to only after poll phase
• Explains potential impact on setImmediate() callback timing

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.59%. Comparing base (9f16142) to head (b41b635).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8126      +/-   ##
==========================================
+ Coverage   76.56%   76.59%   +0.03%     
==========================================
  Files         115      115              
  Lines        9595     9595              
  Branches      322      321       -1     
==========================================
+ Hits         7346     7349       +3     
+ Misses       2248     2245       -3     
  Partials        1        1              

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