Drop support for PHP 7.1

[deguif]

Follow up of #1701

[philkra]

LGTM

[ruflin]

Would this all work but keep this target + the Docker file in? Or will all the tests fail because of the composer requirement?

[deguif]

I think this won't work because of the composer requirement, at least if --ignore-platform-reqs is not used

[ruflin]

A few general questions:

• How widely is 7.1 still used?
• Is it easy for users to upgrade to 7.2 or later?
• What are the things you expect we introduce in Elastica which will not be compatible with 7.1?

I'm good with moving forward with this PR. In the past what I did was that what we test and what is "supported" was not necessarily the same. This meant as soon as we introduced a feature that actually broke a previous PHP version we knew and then removed the testing. Like this it was easy to communicate what broke it.

[deguif]

@ruflin I'm not an expert on the subject but here some stats from composer: https://blog.packagist.com/php-versions-stats-2019-1-edition/

Upgrading to PHP 7.2 should be pretty straightforward.
And some big packages on composer already switched to PHP 7.2 as minimum requirement, see @symfony 5 for example.

I think it could be judicious to really remove PHP 7.1 support as you are about to publish a new major version. This also brings opportunity to boost PHP 7.2 adoption if users of packages need this new version, which is good for the PHP community.

I see two PHP 7.2 features that could be used: object type-hint and basic contravariance.

[ruflin]

@deguif Thanks for sharing the additional details. I especially like the point around boosting adoption of 7.2.