Feature: Build static assets conditionally for Backoffice and Login (#17527)

* feat: conditionally install and build the 'login' and 'backoffice' targets depending on source files

* feat: remove the preconditions target, because we are now compiling a file list to check if we need to build

* feat: remove the 'clean' targets, because the project will be cleaned if any of the compiled file lists do not exist

* feat: remove the preserve.* files as they are no longer needed

* Enable default content items again

* Remove package.json from restore target inputs

* Include generated files as static assets after build

* Clean up project files

* Exclude CS0618 warning as error and fix CA2264

* Exclude CS0612 warning as error

* Suppress removal of test fixture

* Use separate property/item groups for backoffice and login project

---------

Co-authored-by: Ronald Barendse <ronald@barend.se>

tests/Umbraco.Tests.Common/Umbraco.Tests.Common.csproj

@@ -7,15 +7,21 @@
     <IsPackable>true</IsPackable>
     <EnablePackageValidation>$(BaseEnablePackageValidation)</EnablePackageValidation>
   </PropertyGroup>
+  
   <PropertyGroup>
-    <!--  TODO: Fix [CS1998] async, [CSO618] obsolete references, fix [SYSLIB0012] obsolete references,
-          and remove overrides -->
-    <WarningsNotAsErrors>CS1998,CS0618,SYSLIB0012</WarningsNotAsErrors>
+    <!--
+      TODO: Fix and remove overrides:
+      [CS1998] async
+      [CS0618] obsolete references
+      [SYSLIB0012] obsolete references
+    -->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors),CS1998,CS0618,SYSLIB0012</WarningsNotAsErrors>
   </PropertyGroup>
+  
   <ItemGroup>
     <PackageReference Include="AutoFixture.AutoMoq" />
-    <PackageReference Include="Moq" />
     <PackageReference Include="AutoFixture.NUnit3" />
+    <PackageReference Include="Moq" />
     <PackageReference Include="NUnit" />
   </ItemGroup>