docs: update promise rejection section to reflect Node.js 15+ behavior

[Vansh1811]

What this PR does

Updates sections/errorhandling/catchunhandledpromiserejection.md to reflect the current Node.js behavior regarding unhandled promise rejections.

Problem

The documentation currently states that unhandled promise rejections "disappear" and that Node.js warning messages are "obviously not a proper error handling method". This is outdated.

Fixes #1360

Changes

• Updated the One Paragraph Explainer to clearly state that since Node.js 15+, unhandled promise rejections crash the process with exit code 1 — they no longer disappear silently
• Added a version behavior table clarifying:
  • Node.js < 15: prints DEP0018 deprecation warning, process continues
  • Node.js 15+: crashes the process with exit code 1 (same as uncaught exceptions)
• Updated the blog quote section to add a note clarifying that the browser-era behavior described in the quote no longer applies to modern Node.js

References

• Node.js PR that introduced this change: process: Change default --unhandled-rejections=throw nodejs/node#33021
• Node.js 15 changelog: https://nodejs.org/en/blog/release/v15.0.0