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-1/initramfs-full.yaml

@@ -4,5 +4,5 @@ postprocess:
     #!/usr/bin/env bash
     mkdir -p /usr/lib/dracut/dracut.conf.d
     cat > /usr/lib/dracut/dracut.conf.d/30-bootc-tier-1.conf << 'EOF'
-    dracutmodules+=" lvm crypt fips "
+    add_dracutmodules+=" lvm crypt fips "
     EOF