Remove support for TYPO3 v11

[brotkrueml]

For now, this is just a collection of tasks to be done once compatibility for TYPO3 v11 is removed.

• [ ] Remove registration of SCHEMA content object in ext_localconf.php
• [ ] Check for parent constructor in AdminPanel/SchemaModule, also skipping rector rule RemoveParentCallWithoutParentRector can be removed, see https://github.com/brotkrueml/schema/commit/7cd0cccaa26a51df0d4109e01b5d53dec83e4d89
• [ ] Remove conflict with sdbreadcrumb in composer.json and ext_emconf.php
• [ ] Remove compatibility with Symfony 5.4 in composer.json
• [ ] Remove compatibility with Testing Framework 7 in composer.json