Use the server-raspi task name instead of ubuntu-server-raspi

debian/changelog

@@ -1,3 +1,12 @@
+livecd-rootfs (2.724) UNRELEASED; urgency=medium
+
+  * Use the server-raspi task name instead of ubuntu-server-raspi. It's still
+    a mystery to me why ubuntu-desktop-raspi works as expected but the server
+    variant needs to use 'server-raspi', even though both tasks seem
+    implemented in the same way.
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Wed, 09 Jun 2021 11:14:13 +0200
+
 livecd-rootfs (2.723) impish; urgency=medium
 
   [ Steve Langasek ]

live-build/auto/config

@@ -792,7 +792,7 @@ case $PROJECT in
 		OPTS="${OPTS:+$OPTS }--initramfs=none"
 		case "$ARCH+${SUBARCH:-}" in
 			*+raspi)
-				add_task install ubuntu-server-raspi
+				add_task install server-raspi
 				;;
 			armhf*)
 				KERNEL_FLAVOURS=generic-lpae