Rollback changes breaking rolling builds (#60040)

* Revert "CI matrix update: add Ubuntu 21.04 (#59578)"

This reverts commit 127a498.

* Revert "CI matrix: add openSUSE 15.2 (#59574)"

This reverts commit f9025ae.

* Revert "CI matrix change: add Windows.Server.Core.20H2 (#59572)"

This reverts commit 5ab435e.

Author: krwq

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

@@ -36,7 +36,6 @@ jobs:
     - ${{ if eq(parameters.platform, 'Linux_arm64') }}:
       - ${{ if eq(parameters.jobParameters.isFullMatrix, true) }}:
         - (Ubuntu.1804.ArmArch.Open)Ubuntu.1804.ArmArch.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-helix-arm64v8-20210106155927-56c6673
-        - (Ubuntu.2104.Arm64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-21.04-helix-arm64v8-20210922170819-34a2d72
       - ${{ if eq(parameters.jobParameters.isFullMatrix, false) }}:
         - (Ubuntu.1804.ArmArch.Open)Ubuntu.1804.ArmArch.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-16.04-helix-arm64v8-20210106155927-56c6673
 
@@ -59,7 +58,7 @@ jobs:
           - RedHat.7.Amd64.Open
           - SLES.15.Amd64.Open
           - (Fedora.34.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:fedora-34-helix-20210913123654-4f64125
-          - (Ubuntu.2104.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-21.04-helix-amd64-20210922170909-34a2d72
+          - (Ubuntu.1910.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-19.10-helix-amd64-cfcfd50-20191030180623
           - (Debian.10.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/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) }}:
@@ -70,11 +69,10 @@ jobs:
             - SLES.12.Amd64.Open
             - SLES.15.Amd64.Open
             - (Fedora.34.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:fedora-34-helix-20210913123654-4f64125
-            - (Ubuntu.2104.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-21.04-helix-amd64-20210922170909-34a2d72
+            - (Ubuntu.1910.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-19.10-helix-amd64-cfcfd50-20191030180623
             - (Debian.10.Amd64.Open)Ubuntu.1804.Amd64.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:debian-10-helix-amd64-20210304164434-56c6673
             - (Debian.11.Amd64.Open)Ubuntu.1804.Amd64.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:debian-11-helix-amd64-20210304164428-5a7c380
             - (Mariner.1.0.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:cbl-mariner-1.0-helix-20210528192219-92bf620
-            - (openSUSE.15.2.Amd64.Open)ubuntu.1604.amd64.open@mcr.microsoft.com/dotnet-buildtools/prereqs:opensuse-15.2-helix-amd64-20210922211241-3cc5714
           - ${{ if eq(parameters.jobParameters.isFullMatrix, false) }}:
             - (Centos.7.Amd64.Open)Ubuntu.1604.Amd64.Open@mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-mlnet-helix-20210714125435-dde38af
             - RedHat.7.Amd64.Open
@@ -130,11 +128,9 @@ jobs:
             - Windows.81.Amd64.Open
             - Windows.10.Amd64.ServerRS5.Open
             - Windows.10.Amd64.Server19H1.Open
-            - Windows.10.Amd64.Server20H2.Open
             - ${{ if ne(parameters.jobParameters.runtimeFlavor, 'mono') }}:
               - (Windows.Nano.1809.Amd64.Open)windows.10.amd64.serverrs5.open@mcr.microsoft.com/dotnet-buildtools/prereqs:nanoserver-1809-helix-amd64-08e8e40-20200107182504
               - (Windows.Server.Core.1909.Amd64.Open)windows.10.amd64.server20h1.open@mcr.microsoft.com/dotnet-buildtools/prereqs:windowsservercore-2004-helix-amd64-20200904200251-272704c
-              - (Windows.Server.Core.20H2.Amd64.Open)windows.10.amd64.serverrs5.open@mcr.microsoft.com/dotnet-buildtools/prereqs:windowsservercore-20H2-helix-amd64-20210922171304-edc3733
           - ${{ if ne(parameters.jobParameters.isFullMatrix, true) }}:
             - Windows.81.Amd64.Open
             - Windows.10.Amd64.Server19H1.ES.Open