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.Benchmarks/Umbraco.Tests.Benchmarks.csproj

@@ -5,11 +5,18 @@
     <IsPackable>false</IsPackable>
     <GenerateProgramFile>false</GenerateProgramFile>
   </PropertyGroup>
+  
   <PropertyGroup>
-    <!--  TODO: Fix [SYSLIB0021] API supports obsolete serialization, [IDE0060] removed unused parameters,
-          [CS0618] update obsolete references, CS0649 field not assigned too, and remove overrides -->
-    <WarningsNotAsErrors>SYSLIB0021,IDE0060,CS0618,CS0649</WarningsNotAsErrors>
+    <!--
+      TODO: Fix and remove overrides:
+      [SYSLIB0021] API supports obsolete serialization
+      [IDE0060] removed unused parameters
+      [CS0618] update obsolete references
+      [CS0649] field not assigned too
+    -->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors),SYSLIB0021,IDE0060,CS0618,CS0649</WarningsNotAsErrors>
   </PropertyGroup>
+  
   <ItemGroup>
     <PackageReference Include="BenchmarkDotNet" />
     <PackageReference Include="Microsoft.Extensions.Logging.Debug" />