Update Ix.NET dependencies (#2249)

* Upgrade to Nerdbank.GitVersioning 3.8.118
* Upgrade to IsExternalInit 1.0.3
* Update to BenchmarkDotNet 0.15.4

Tried to upgrade to xunit 2.9.3 but this started producing weird assembly resolution failures in .NET FX for `System.Runtime.CompilerServices.Unsafe`. Will try that again after we've removed all dependencies on `net6.0` for the next release.

Ix.NET/Source/Benchmarks.System.Interactive/Benchmarks.System.Interactive.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <Optimize>true</Optimize>
     <Configurations>Current Sources;Ix.net 3.1.1;Ix.net 3.2</Configurations>
   </PropertyGroup>
@@ -20,7 +20,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.13.1" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.15.4" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(Configuration)|$(Platform)'=='Ix.net 3.1.1|AnyCPU'">

Ix.NET/Source/Directory.Build.props

@@ -30,7 +30,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.1.1" PrivateAssets="All"/>
-    <PackageReference Include="Nerdbank.GitVersioning" Version="3.4.255" PrivateAssets="all" />
+    <PackageReference Include="Nerdbank.GitVersioning" Version="3.8.118" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup>

Ix.NET/Source/System.Linq.Async.SourceGenerator/System.Linq.Async.SourceGenerator.csproj

@@ -6,6 +6,6 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" PrivateAssets="all" />
-    <PackageReference Include="IsExternalInit" Version="1.0.2" PrivateAssets="all" />
+    <PackageReference Include="IsExternalInit" Version="1.0.3" PrivateAssets="all" />
   </ItemGroup>
 </Project>