v2.1-RC1 PHP8.1 support

• Adds support for PHP8.1
• Improve tests and make them run on PHP8.1

v2.0-RC1 PHP8 support

• Remove a lot of PHP5 related cruft
• Possibly breaks compact with PHP < 7.4
• Adds support for PHP8
• Makes tests run on PHP7 and PHP8
• Tests will likely not run on Windows anymore
• Only tested with PHP7.4.16 and PHP8.0.3 on Debian buster (see Dockerfile)
• Only tested against Cassandra 3.11.10