Add a composer conflict for symfony/dom-crawler >=4 to 8.7 only.

Problem/Motivation

symfony/dom-crawler is an indirect dependency of drupal core. We ship with version 3.4.x in our lock file (the highest versions compatible with php 7.0), however if composer upgrade is ran in php 7.1, then symfony 4 versions can be installed.

dom-crawler 4.3 introduced a new means of html parsing that revealed some broken html output on certain settings forms, which caused problems with functional testing. We've fixed the actual problem in 8.8 in the parent issue, however the change was deemed to disruptive to backport to a patch release.

Proposed resolution

To prevent the test failures on 8.7 without disruption, we should add a conflict with dom-crawler 4+ in our composer.json.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet