ci: #3636958 Run Webform 6.2.x CI on PHP 8.4

Jacob Rockowitzrequested to merge
issue/webform-3636958:3636958-update-webform-6.2.x into 6.2.x

AI-assisted by Codex

Summary

Set PHP_VERSION: '8.4' for the Webform 6.2.x Composer CI job so its development dependencies can be installed.

Validation

The pipeline will verify Composer installation and downstream jobs.

AI-Generated: Yes (Used Codex to prepare the CI configuration change and merge request.)

Closes #3636958

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