Move ARM and ARM64 to Ubuntu 18.04

.azure/pipelines/ci.yml

@@ -565,7 +565,7 @@ stages:
       jobDisplayName: "Build: Linux Musl ARM"
       agentOs: Linux
       useHostedUbuntu: false
-      container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm-alpine-20210409142327-044d5b9
+      container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-arm-alpine-20210409142425-044d5b9
       buildScript: ./eng/build.sh
       buildArgs:
         --arch arm
@@ -601,7 +601,7 @@ stages:
       jobDisplayName: "Build: Linux Musl ARM64"
       agentOs: Linux
       useHostedUbuntu: false
-      container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-cross-arm64-alpine-20200413125008-406629a
+      container: mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-cross-arm64-alpine-20210409142425-b2c2436
       buildScript: ./eng/build.sh
       buildArgs:
         --arch arm64