March 2023 minor release

This minor release was made following on from the documentation being finalised, after everything has been tested with real usage.

The only functionality introduced/changed is in regards to the header/write function callback usage - now, if there is an existing write function, a new one will not be added when adding a Curl to a CurlMulti.

Read about concurrent curl requests here: https://github.com/PhpGt/Curl/wiki/Concurrent-Curl-requests