Target only net7.0 during source-build - Port of #75168 to main (#77052)

* Target only net7.0 during source-build
Port of #75168 to main.

* Target net7.0 in additional projects

* Update compatibility suppressions

* Only target net7.0 for source-build

* Disable package validation for source-build

* Revert compatibility suppression changes

* Use NetCoreAppCurrent instead of hardcoding net7.0

* Revert mono Apple TFM change

* Remove unnecessary LangVersion

* Address code review comments.

Co-authored-by: Logan Bussell <loganbussell@microsoft.com>

Author: crummel

Directory.Build.props

@@ -53,6 +53,7 @@
     <NetCoreAppCurrentBrandName>.NET $(NetCoreAppCurrentVersion)</NetCoreAppCurrentBrandName>
     <NetCoreAppCurrent>net$(NetCoreAppCurrentVersion)</NetCoreAppCurrent>
     <NetCoreAppMinimum>net6.0</NetCoreAppMinimum>
+    <NetCoreAppMinimum Condition="'$(DotNetBuildFromSource)' == 'true'">$(NetCoreAppCurrent)</NetCoreAppMinimum>
 
     <NetCoreAppToolCurrentVersion>7.0</NetCoreAppToolCurrentVersion>
     <NetCoreAppToolCurrent>net$(NetCoreAppToolCurrentVersion)</NetCoreAppToolCurrent>

eng/SourceBuild.props

@@ -48,6 +48,7 @@
       <InnerBuildArgs>$(InnerBuildArgs) /p:BuildDebPackage=false</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) /p:EnableNgenOptimization=false</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) /p:AdditionalRuntimeIdentifierParent=$(BaseOS)</InnerBuildArgs>
+      <InnerBuildArgs>$(InnerBuildArgs) /p:EnablePackageValidation=false</InnerBuildArgs>
     </PropertyGroup>
   </Target>