add glibc-gconv-extra to Rocky Linux 9 + 10 containers

[boegel]

Required for libxml2 tests, popped up when trying to install libxml2-2.14.3-GCCcore-14.3.0.eb in Rocky Linux 9 container:

./test/errors/759398.xml:1: parser error : Unsupported encoding: ISO-8859-5
<?xml version='1.0' encoding='ISO-8859-5' standalone='no'?>
                                         ^
./test/errors/759398.xml : failed to parse

No need to add to Rocky Linux 8 container, since it's already there (as a dependency for something else, probably):

$ apptainer shell docker://ghcr.io/easybuilders/rockylinux-8.10
Apptainer> rpm -qa | grep glibc-gconv
glibc-gconv-extra-2.28-251.el8_10.25.x86_64

[boegel]

This also fixes a problem when building gettext-0.22.5.eb and gettext-0.25.eb in the Rocky Linux 10 container:

iconv-ostream.c:311:3: error: initialization of ‘void (*)(struct any_ostream_representation *, ostream_flush_scope_t)’ from incompatible pointer type ‘void (*)(struct any_ostream_representation *)’ [-Wincompatible-pointer-types]
  311 |   iconv_ostream__flush,
      |   ^~~~~~~~~~~~~~~~~~~~
iconv-ostream.c:311:3: note: (near initialization for ‘iconv_ostream_vtable.flush’)
make[4]: *** [Makefile:6192: config.h] Error 1

see also spack/spack#44320