Since 1.36, intl has been a required PHP extension, and PHP 7.4 dropped
support for ICU < 50.1 (Unicode 6.2), so:
* In SpecialVersion, don't check whether INTL_ICU_VERSION is defined.
* Remove check in the installer for outdated Unicode normalization. It
was added over twelve years ago in r70126 (a21fb8651f) with a
comment that it should be kept up to date, but no one ever did.
* Remove IcuCollation::getUnicodeVersionForICU(), which contained a long
list of ICU versions and corresponding Unicode versions that had to be
kept up to date manually. Instead use IntlChar::getUnicodeVersion(),
which was added in PHP 7.0. There are no known callers outside core.
* Remove LinkFilter::supportsIDN(), as ICU has had support for UTS#46
since version 4.6. There are no known callers outside core. Also
remove $flags and $variant from the idn_to_utf8() call, which match
PHP 7.4's defaults. (INTL_IDNA_VARIANT_2003 was the default in 7.3.)
* Display the ICU and Unicode versions in the installer, just below the
PHP version. The ICU version is shown on Special:Version near the PHP
version, and it probably makes sense to show it there as well.
Change-Id: Ibdfac1a6f46fd56b84de1140292e0ec863f043ee
8c4cea2ca2