Remove hardcoded System.Security.Cryptography.Xml version (#48029)

Author: Tratcher

eng/SourceBuildPrebuiltBaseline.xml

@@ -19,6 +19,9 @@
     <UsagePattern IdentityGlob="Microsoft.CodeAnalysis.Workspaces.Common/*3.3.1*" />
     <UsagePattern IdentityGlob="System.Composition/*1.0.31*" />
     <UsagePattern IdentityGlob="System.Threading.Tasks.Extensions/*4.5.3*" />
+    
+    <!-- Added to unblock dependency flow, needs review. -->
+    <UsagePattern IdentityGlob="System.Security.Cryptography.Xml/*6.0.0*" />
 
     <!-- These are coming in via runtime but the source-build infra isn't able to automatically pick up the right intermediate. -->
     <UsagePattern IdentityGlob="Microsoft.NET.ILLink.Tasks/*8.0.*" />

eng/tools/RepoTasks/RepoTasks.csproj

@@ -23,8 +23,6 @@
     <PackageReference Include="Microsoft.Build.Framework" Version="$(MicrosoftBuildFrameworkVersion)" />
     <PackageReference Include="Microsoft.Build.Tasks.Core" Version="$(MicrosoftBuildTasksCoreVersion)" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" Version="$(MicrosoftBuildUtilitiesCoreVersion)" />
-    <!-- Manually updated version from 6.0.0 to address CVE-2021-43877 -->
-    <PackageReference Include="System.Security.Cryptography.Xml" Version="$(RepoTasksSystemSecurityCryptographyXmlVersion)" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'net472'">