Use 8.0 dependencies for non net9.0, except for System.Diagnostics.DiagnosticSource

Author: paulomorgado

Directory.Packages.props

@@ -9,7 +9,9 @@
         possible and only deviate (use a specific version) when a package has a
         more specific patch which must be reference directly.
     -->
-    <LatestRuntimeOutOfBandVer>9.0.0</LatestRuntimeOutOfBandVer>
+    <Runtime9OutOfBandVer>9.0.0</Runtime9OutOfBandVer>
+    <LatestRuntimeOutOfBandVer>8.0.0</LatestRuntimeOutOfBandVer>
+    <LatestRuntimeOutOfBandVer Condition="'$(TargetFramework)' == 'net9.0'">$(Runtime9OutOfBandVer)</LatestRuntimeOutOfBandVer>
 
     <!-- Mitigate https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-43485. -->
     <SystemTextEncodingsWebOutOfBandMinimumCoreAppVer>8.0.0</SystemTextEncodingsWebOutOfBandMinimumCoreAppVer>
@@ -59,7 +61,7 @@
         3) The .NET runtime team provides extra backward compatibility guarantee to System.Diagnostics.DiagnosticSource
           even during major version bumps, so compatibility is not a concern here.
     -->
-    <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="$(LatestRuntimeOutOfBandVer)" />
+    <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="$(Runtime9OutOfBandVer)" />
 
     <!--
         We use conservative versions of these packages where an upgrade might

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OpenTelemetry.Exporter.OpenTelemetryProtocol.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>$(TargetFrameworksForLibrariesExtended)</TargetFrameworks>
     <Description>OpenTelemetry protocol exporter for OpenTelemetry .NET</Description>
@@ -17,6 +17,7 @@
     and https://github.com/dotnet/runtime/issues/92509 -->
     <NoWarn>$(NoWarn);SYSLIB1100;SYSLIB1101</NoWarn>
     <AllowUnsafeBlocks Condition="'$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == '$(NetFrameworkMinimumSupportedVersion)'">true</AllowUnsafeBlocks>
+    <ImplicitUsings>enable</ImplicitUsings>
   </PropertyGroup>
 
   <ItemGroup>
@@ -25,6 +26,7 @@
 
   <ItemGroup>
     <Reference Include="System.Net.Http" Condition="'$(TargetFramework)' == '$(NetFrameworkMinimumSupportedVersion)'" />
+    <Using Include="System.Net.Http" Condition="'$(TargetFramework)' == '$(NetFrameworkMinimumSupportedVersion)'" />
   </ItemGroup>
 
   <ItemGroup>