Fixes for alpine 3.16

[cedric-anne]

1. gnu-libiconv is now installed on base image, no need to install it manually (see Use gnu-libiconv on alpine only for php iconv extension docker-library/php#1264).

2. ICU data has been splitted in alpine 3.16 and icu-data-full has to be installed to have a support of many charsets (see https://wiki.alpinelinux.org/wiki/Release_Notes_for_Alpine_3.16.0#ICU_data_split). Without this, our test suite fails:

=> tests\units\Toolbox::testFilename():
In file /var/glpi/tests/functionnal/Toolbox.php on line 139, string() failed for data set [7] of data provider filenameProvider: string(8) '24.1.txt' is not identical to string(90) '24.1-zhang-wen-jian-ming-jiang-dao-zhi-nei-rong-chu-zhi-biao-tou-zhong-de-lian-xu-xing.txt'
-Expected
+Actual
@@ -1 +1 @@
-string(90) "24.1-zhang-wen-jian-ming-jiang-dao-zhi-nei-rong-chu-zhi-biao-tou-zhong-de-lian-xu-xing.txt"
+string(8) "24.1.txt"

3. Some instructions related to PHP 7.4 were still present.

4. gettext package is up-to-date in alpine 3.16, no need to fetch it from an alternative repository.