CI: update build architectures

- armvirt target has been renamed to armsr (Arm SystemReady) [1].
- armsr-armv8 has been switched to aarch64_generic architecture [2].

1. https://github.com/openwrt/openwrt/commit/40b02a230167626def69389452f19b7109aaeac1
2. https://github.com/openwrt/openwrt/commit/e0f06ddc23b2503a1791ae7e97b02e2647e8a70d

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>

.github/workflows/multi-arch-test-build.yml

@@ -15,12 +15,12 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - arch: aarch64_cortex-a53
-            target: mvebu-cortexa53
+          - arch: aarch64_generic
+            target: armsr-armv8
             runtime_test: true
 
           - arch: arm_cortex-a15_neon-vfpv4
-            target: armvirt-32
+            target: armsr-armv7
             runtime_test: true
 
           - arch: arm_cortex-a9_vfpv3-d16