Remove support for TYPO3 v9

[brotkrueml]

For now, this is just a collection of tasks to be done once compatibility for TYPO3 v9 is removed. It is likely, that the support is dropped, when the extension will be made compatible with TYPO3 v12.0.

• Use constant TYPO3 instead of TYPO3_MODE and set minimum TYPO3 version to 10.4.11
• Remove translations other than English
• The documentation is adjusted
• A changelog entry is available.