Deprecate and ignore $version parameter of curl_version()

Deprecate and ignore $version parameter of curl_version()

`curl_version()`[1] (of ext/curl) makes `curl_version_info()`[2] (of
libcurl) available to PHP userland.  The latter requires to pass an
`age` argument which usually is `CURLVERSION_NOW`, so that the
information returned by the runtime matches the declarations used
during compile time.  For C programs it is simply necessary to pass
this information, and in rare occasions it might make sense to pass
something else than `CURLVERSION_NOW`.  curl.h notes:

| The 'CURLVERSION_NOW' is the symbolic name meant to be used by
| basically all programs ever that want to get version information.

For the PHP binding, using a newer `age` than available at compile time
will neither provide the PHP program more information, nor would using
an older `age` have tangible benefits.

We therefore deprecate the useless `$version` parameter, and if it is
passed nonetheless, we use `CURLVERSION_NOW` instead of the supplied
value, and raise a warning.

[1] <https://www.php.net/manual/en/function.curl-version.php>
[2] <https://curl.haxx.se/libcurl/c/curl_version_info.html>