initramfs: Switch to add_dracutmodules

Per discussion in https://github.com/rhkdump/kdump-utils/pull/29#issuecomment-2303932537

This would have avoided kdump breakage, and the original motivation
of avoiding dracut error spam from missing things is gone now
that Fedora is using dracut-ng which has
https://github.com/dracut-ng/dracut-ng/commit/d73cc24e112c01aa701a96a7b8a58adce78409e7

tier-0/initramfs.yaml

@@ -6,13 +6,13 @@ postprocess:
     cat > /usr/lib/dracut/dracut.conf.d/20-bootc-base.conf << 'EOF'
     # We want a generic image; hostonly makes no sense as part of a server side build
     hostonly=no
-    dracutmodules+=" kernel-modules dracut-systemd systemd-initrd base ostree "
+    add_dracutmodules+=" kernel-modules dracut-systemd systemd-initrd base ostree "
     EOF
     cat > /usr/lib/dracut/dracut.conf.d/22-bootc-generic.conf << 'EOF'
     # Extra modules that we want by default that are known to exist in the kernel
-    dracutmodules+=" virtiofs "
+    add_dracutmodules+=" virtiofs "
     EOF
     cat > /usr/lib/dracut/dracut.conf.d/49-bootc-tpm2-tss.conf << 'EOF'
     # We want this for systemd-cryptsetup tpm2 locking
-    dracutmodules+=" tpm2-tss "
+    add_dracutmodules+=" tpm2-tss "
     EOF