ci: use ubuntu-24.04 runners for ARM64 build availability

ubuntu-26.04 and ubuntu-26.04-arm64 are in public preview with limited
runner pool capacity, causing ARM64 jobs to queue indefinitely. Switch to
ubuntu-24.04 and ubuntu-24.04-arm64 (GA) which have full capacity.

The host OS version has no effect on build output since sdkcraft runs
inside LXD.

Author: rajannpatel

.github/workflows/upload.yml

@@ -21,9 +21,9 @@ jobs:
       matrix:
         include:
           - platform: '["amd64"]'
-            runner: '["ubuntu-26.04"]'
+            runner: '["ubuntu-24.04"]'
           - platform: '["arm64"]'
-            runner: '["ubuntu-26.04-arm64"]'
+            runner: '["ubuntu-24.04-arm64"]'
     uses: canonical/sdkcraft-actions/.github/workflows/upload.yml@main
     with:
       platforms: ${{ matrix.platform }}