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>

tools/Umbraco.JsonSchema/Umbraco.JsonSchema.csproj

@@ -4,11 +4,18 @@
     <IsPackable>false</IsPackable>
     <EnablePackageValidation>false</EnablePackageValidation>
   </PropertyGroup>
+  
   <PropertyGroup>
-    <!--  TODO: Fix [SA1117] params all on same line, [SA1401] make fields private,
-          [SA1134] own line attributes, [CS9042] not obsolete, and remove overrides -->
-    <WarningsNotAsErrors>SA1117,SA1401,SA1134,CS9042</WarningsNotAsErrors>
+    <!--
+      TODO: Fix and remove overrides:
+      [SA1117] params all on same line
+      [SA1401] make fields private
+      [SA1134] own line attributes
+      [CS9042] not obsolete
+    -->
+    <WarningsNotAsErrors>$(WarningsNotAsErrors),SA1117,SA1401,SA1134,CS9042</WarningsNotAsErrors>
   </PropertyGroup>
+  
   <ItemGroup>
     <PackageReference Include="CommandLineParser" VersionOverride="2.9.1" />
     <PackageReference Include="NJsonSchema" VersionOverride="11.0.2" />