docs: update redirecting url

[samford]

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A php.net URL in docs/Versions.md is redirecting to www.php.net, so this updates the URL to avoid the redirection. I noticed this because php.net was having server issues (timing out) and the docs job on brew CI was failing with the following:

For the Links > External check, the following failures were found:

* At ./_site/Versions.html:65:

  External link https://php.net/supported-versions.php failed: got a time out (response code 301) (status code 301)


HTML-Proofer found 1 failure!

The server issue was addressed upstream, so this PR just addresses the redirection (which isn't a failure now that the server is available again).