Use F#9 for the main lib

Author: gusty

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.0",
+    "version": "9.0.0",
     "rollForward": "latestFeature",
     "allowPrerelease": true
   },

src/FSharpPlus/FSharpPlus.fsproj

@@ -21,15 +21,15 @@
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
     <Configurations>Debug;Release;Fable;Test</Configurations>
     <Platforms>AnyCPU</Platforms>
-    <LangVersion>8.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <LangVersion Condition=" '$(Configuration)' == 'Fable' ">6.0</LangVersion>
 
     <DefineConstants Condition=" '$(Configuration)' == 'Test'">$(DefineConstants);TEST_TRACE</DefineConstants>
     <DefineConstants Condition=" '$(Configuration)' == 'Fable'">$(DefineConstants);FABLE_COMPILER</DefineConstants>
     <DefineConstants Condition=" '$(Configuration)' == 'Fable4'">$(DefineConstants);FABLE_COMPILER;FABLE_COMPILER_4</DefineConstants>
     <TargetFramework>net8.0</TargetFramework>
     <!--<OutputPath>..\..\bin</OutputPath>-->
-  </PropertyGroup>
+    </PropertyGroup>
   <ItemGroup>
     <Compile Include="Internals.fs" />
     <Compile Include="Extensions/Option.fs" />
@@ -125,8 +125,4 @@
 		<Content Include="Math/*.fs" PackagePath="fable/Math" />
 		<Content Include="Data/*.fs" PackagePath="fable/Data" />
 	</ItemGroup>
-
-	<ItemGroup>
-		<PackageReference Update="FSharp.Core" Version="6.0.6" />
-	</ItemGroup>
 </Project>