upgrade prophecy to 1.20

Problem/Motivation

Last part of dependency updates for PHP 8.4 compatibility #3427903: [META] Make Drupal 10/11 compatible with PHP 8.4

Ref https://github.com/phpspec/prophecy/issues/624

Steps to reproduce

composer update phpspec/* -W

Proposed resolution

Upgrade to 1.20.0

Remaining tasks

upgrade and backport to 10.4.x

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet