fix: remove deprecated curl_close() calls

[mogliabr]

Problem

This package requires PHP ^8.2 (see composer.json), and curl_close()
is deprecated as of PHP 8.5. Running on 8.5 emits deprecation notices from
the curl_close() calls in the provider.

Why removing is safe

curl_close() has been a no-op since PHP 8.0: the CurlHandle is freed
automatically by the garbage collector once it goes out of scope. Because
the package already targets ^8.2, there is no PHP 7 path where an
explicit close is required, so the calls can be removed outright with no
change in behavior.

Change

Removes the curl_close() calls.

Compatibility note

Since this package targets ^8.2, removing the calls is the right change.
If a downstream fork ever needs to support PHP 7, keep the explicit close
behind a version check rather than removing it:

private function closeCurl($handle): void
{
    if (PHP_VERSION_ID < 80000) {
        curl_close($handle);
    }
}