arm-systemready/ir-acs: Update URL

The github URL where the image was located has gone away on the master
branch.  Update the URL to point to the legacy branch, which should stay
around (according to the documentation).

Fixes: aebe535aa8c3 ("arm-systemready: Introduce the Arm SystemReady layer")
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-systemready/recipes-test/arm-systemready-acs/arm-systemready-ir-acs.bb

@@ -48,7 +48,7 @@ TEST_SUITES = "arm_systemready_ir_acs"
 PV = "2.0.0"
 PV_DATE = "23.03"
 FULL_PV = "v${PV_DATE}_${PV}"
-ARM_SYSTEMREADY_IR_ACS_BRANCH ?= "main"
+ARM_SYSTEMREADY_IR_ACS_BRANCH ?= "legacy_systemready"
 IMAGE_FILENAME = "ir-acs-live-image-generic-arm64-${PV}.wic"
 SRC_URI = " \
     https://github.com/ARM-software/arm-systemready/raw/${ARM_SYSTEMREADY_IR_ACS_BRANCH}/IR/prebuilt_images/${FULL_PV}/ir-acs-live-image-generic-arm64.wic.xz;name=acs-img;downloadfilename=${IMAGE_FILENAME}.xz \