archiso: fix deprecated boot modes and grub/systemd-boot conflict

Replace five deprecated boot mode names with the canonical 'bios.syslinux'
and 'uefi.systemd-boot', removing the ia32 grub mode that required grub
installed and conflicted with systemd-boot.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

setup/archiso/overlay/profiledef.sh

@@ -7,7 +7,7 @@ iso_application="M-Archy Arch Linux Installer"
 iso_version="$(date --date="@${SOURCE_DATE_EPOCH:-$(date +%s)}" +%Y.%m.%d)"
 install_dir="arch"
 buildmodes=('iso')
-bootmodes=('bios.syslinux.mbr' 'bios.syslinux.eltorito' 'uefi-ia32.grub.esp' 'uefi-x64.systemd-boot.esp' 'uefi-x64.systemd-boot.eltorito')
+bootmodes=('bios.syslinux' 'uefi.systemd-boot')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"