Bump ICU version requirement

[cmb69]

Description

The last time this has been done for PHP 7.4[1]; since then the requirement is ICU >= 50.1, which has been released 12 years ago. In my opinion, we should bump the requirement (not sure exactly to which version though). That may allow us to drop some conditional compilation, and also some test cases for old ICU versions. E.g.

php-src/ext/intl/tests/collator_get_sort_key_variant2.phpt

Line 7 in f5e743a

<?php if (version_compare(INTL_ICU_VERSION, '53.1') >= 0) die('skip for ICU < 53.1'); ?>

Note that ICU 53.1 has been released more than 10 years ago.

[1] 20fa2e7

[devnexen]

I would say at least 57.1 as Solaris 11 can get it from opencsw. We, normally, plan to drop centos 7 support for the next major php release and centos 8 it s 60.3 but might be a bit too "recent" tough (2019).

[cmb69]

Okay, I'll work on a PR.