Bump required libcurl version to 7.17.1

Hi all, quite a while ago I've submitted <https://github.com/php/php-src/pull/4917>, but received no feedback on that so far, so I'm writing this mail. The pull request is about bumping the required libcurl version for ext/curl to 7.17.1, which has been released twelve years ago. The main reason for the bump is to remove most of the obsolete constants which have been scheduled for removal from libcurl for more than ten years. The fact that we still use them, has almost bitten us not long ago[1], so I think that PHP 8 is a good opportunity to finally remove them. If there are no objections, I'll merge PR #4917 in a week. [1] <https://github.com/curl/curl/pull/4601> Thanks, Christoph