CI matrix update: add Ubuntu 21.04 (#59578)

Author: Jan Jahoda

eng/pipelines/libraries/helix-queues-setup.yml

@@ -36,6 +36,7 @@ jobs:
     - ${{ if eq(parameters.platform, 'Linux_arm64') }}:
       - ${{ if eq(parameters.jobParameters.isFullMatrix, true) }}:
         - (Ubuntu.1804.ArmArch.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-16.04-helix-arm64v8-20210106155927-56c6673
+        - (Ubuntu.2104.Arm64.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-21.04-helix-arm64v8-20210922170819-34a2d72
       - ${{ if eq(parameters.jobParameters.isFullMatrix, false) }}:
         - (Ubuntu.1804.ArmArch.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-16.04-helix-arm64v8-20210106155927-56c6673
 
@@ -58,7 +59,7 @@ jobs:
           - RedHat.7.Amd64.Open
           - SLES.15.Amd64.Open
           - (Fedora.34.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:fedora-34-helix-20210913123654-4f64125
-          - (Ubuntu.1910.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-19.10-helix-amd64-cfcfd50-20191030180623
+          - (Ubuntu.2104.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-21.04-helix-amd64-20210922170909-34a2d72
           - (Debian.10.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:debian-10-helix-amd64-bfcd90a-20200121150006
         - ${{ if or(ne(parameters.jobParameters.testScope, 'outerloop'), ne(parameters.jobParameters.runtimeFlavor, 'mono')) }}:
           - ${{ if eq(parameters.jobParameters.isFullMatrix, true) }}:
@@ -69,7 +70,7 @@ jobs:
             - SLES.12.Amd64.Open
             - SLES.15.Amd64.Open
             - (Fedora.34.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:fedora-34-helix-20210913123654-4f64125
-            - (Ubuntu.1910.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-19.10-helix-amd64-cfcfd50-20191030180623
+            - (Ubuntu.2104.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:ubuntu-21.04-helix-amd64-20210922170909-34a2d72
             - (Debian.10.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:debian-10-helix-amd64-20210304164434-56c6673
             - (Debian.11.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:debian-11-helix-amd64-20210304164428-5a7c380
             - (Mariner.1.0.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:cbl-mariner-1.0-helix-20210528192219-92bf620