Remove support of PHP 7.2 and 7.3

[brotkrueml]

By now, PHP 7.2, 7.3 and 7.4 is supported. End of 2020 PHP drops the support of version 7.2 and PHP 7.3 will be moved to security fixes only. The support of the PHP version 7.2 and 7.3 are therefore removed in schema version 2.0. Only PHP 7.4 and 8.0 are supported.

• The minimum version of PHP is adjusted in ext_emconf.php and composer.json.
• PHP 7.2 and 7.3 are removed from Github Actions CI
• The PHP 7.3 and PHP 7.4 language features are used throughout the code.
• The documentation and README.md reflect this change.
• An entry to the CHANGELOG.md is available.