Update minimum required PHP version to 5.3.23

Per discussion on the ML, and per technical issues we've been attempting to
resolve within the framework.

5.3.23 was chosen for the following reasons:

- 5.3.9 and up have a fix that allows a class to implement multiple interfaces
  that define the same method, so long as the signatures are compatible. Prior
  to that version, doing so raised a fatal error. This change is necessary in
  order to solve a problem with separated interface usage in the framework.

- 5.3.23 contains a [PHP bug #62672](https://bugs.php.net/bug.php?id=52861).
  Adopting this version or greater will allow us to (eventually) remove polyfill
  support that works around the symptoms of that issue.

There were no compelling fixes in later versions of the 5.3 series that made
sense, other than 5.3.27, as the last 5.3 version released.