Rename form element due to conflict with TYPO3 v12.3+

[brotkrueml]

With TYPO3 v12.3 a new form element "CountrySelect" was introduced:
https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/12.3/Feature-99735-NewCountrySelectFormElement.html

The name of Core's form element is the same as in this extension which rises problems. In the long run Core's form element should be used. However, for migration it may be useful to have this form element also available in TYPO3 v12.

• Remove compatibility of this extension with TYPO3 v12 in version 2.
• Rename the form element in version 3, so both form elements can be installed and used in one installation.
• Adjust the documentation to reflect this change.
• Add changelog entries.