COMDOX-1448: Backward incompatible changes highlights for 2.4.9-beta

[dshevtsov]

Purpose of this pull request

This pull request adds documentation for the major backward-incompatible changes introduced in the 2.4.9-beta release of Adobe Commerce and Magento Open Source. The Backward-incompatible changes highlights page is updated with a new 2.4.9-beta section that describes impact and required actions for each change.

Summary of changes:

• Added a 2.4.9-beta section to the backward-incompatible changes highlights page with seven major changes:
  • GraphQL alias limit validation — validation to limit the number of aliases in GraphQL requests (default 10); configurable in Admin.
  • GraphQL query length validation — configurable query length limit (default ~1 MB) to reduce DoS risk.
  • New 2FA blocks for identity verification — new blocks for configuring multiple 2FA providers (Magento_TwoFactorAuth).
  • New Relic migration from REST v2 to NerdGraph — Admin configuration steps for NerdGraph API (Magento_NewRelicReporting).
  • Symfony 7.4 LTS support — type and method signature alignment for custom code extending Symfony classes.
  • Valkey 8.x CLI command — new CLI option for Valkey configuration.
  • Zend_Cache replaced with symfony/cache — migration notes for extensions that depend on Zend_Cache.

Each subsection includes Impact and Action required so merchants and extension developers can plan upgrades.

Affected pages

• https://developer.adobe.com/commerce/php/development/backward-incompatible-changes/