End of life plans for php 7.0 & deprecate php 7.1

Php 7.1 is unsupported as of the start of last month. We have been saying we will remove php 7.0 support for a while - we should announce end of support for php 7.0 and start to message people to get off php 7.1. This went OK for php 5.6 (which actually still reports more installs that 7.0 oddly).

I propose we

1. publish a blog saying that the March release of 5.23 will be the last release supporting php 7.0
2. offer support for 7.0 in 5.21 for the duration of that being the ESR (as we did for 5.6)
3. plan to discontinue support for 7.1 in the months after the next ESR is released (e.g Sep 2020) - with longer support for it in the ESR
4. add in app notifications for people to get off 7.1

• Our recommended version should be php 7.3 which works well.
• We are seeing increasing issues with composer packages dropping support for php 7.0 & 7.1. I think this is mostly because of phpunit issues, which have also been hitting us.
• In practice my guess is that CI aside the code will work fine on php 7.0 but it just won't be supported.