Drop support for PHP 7.1

[derrabus]

With the sunset of Symfony 4, we don't need to support PHP 7.1 anymore.

This PR proposes to bump all polyfill packages to PHP 7.2. This also applies to symfony/polyfill-php72 which basically becomes an empty package now and can be removed from the subtree split after its next release.

[nicolas-grekas]

🤔 I don't think we need to do this. It's expected for polyfills to support old versions to me. + looking at the diff, this look unneeded, from a technical pov.

Instead, we could decide that new polyfills shouldn't support PHP 7. That'd allow making them more accurate, thanks to better type declarations.

[derrabus]

Sure, we don't need to. I just thought, it would make #426 and possibly other future contributions easier.

[stof]

Well, "future contributions" being easier depend on which PHP version changed the behavior of a function (as that's what made #426 require a different handling for PHP 7.1 vs 7.2+). And the code needed to support PHP 7.1 was still easy IMO.

I would not do this min version bump for now. I totally expect polyfills to support old PHP versions.

[derrabus]

All right, let's not.