[Sync EN] Add note to shell_exec that the backtick operator is deprecated (#5719)

[github-actions]

Commit: https://github.com/php/doc-en/commit/81593f08db3320d5366bf925c8561f097a9d69ad
PR: https://github.com/php/doc-en/pull/5719

Archivos ES a actualizar

• reference/exec/functions/shell-exec.xml