eng: suppress NETSDK1212 warnings by conditioning IsTrimmable on net6.0+

The <IsTrimmable> property is only meaningful for net6.0+ TFMs;
applying it unconditionally to netstandard2.0 targets produces
NETSDK1212 build warnings in every CI run. Add an MSBuild condition
so IsTrimmable is only set when the target framework is compatible
with net6.0.

Affected projects:
  FSharp.Data.Csv.Core, FSharp.Data.Html.Core, FSharp.Data.Http,
  FSharp.Data.Json.Core, FSharp.Data.Runtime.Utilities,
  FSharp.Data.WorldBank.Core, FSharp.Data.Xml.Core

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/FSharp.Data.Csv.Core/FSharp.Data.Csv.Core.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="CsvRuntime.fs" />

src/FSharp.Data.Html.Core/FSharp.Data.Html.Core.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
     <DefineConstants>$(DefineConstants);HIDE_REPRESENTATION</DefineConstants>
   </PropertyGroup>
   <ItemGroup>

src/FSharp.Data.Http/FSharp.Data.Http.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="Http.fs" />

src/FSharp.Data.Json.Core/FSharp.Data.Json.Core.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="JsonValue.fs" />

src/FSharp.Data.Runtime.Utilities/FSharp.Data.Runtime.Utilities.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="PublicApi.fs" />

src/FSharp.Data.WorldBank.Core/FSharp.Data.WorldBank.Core.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="WorldBankRuntime.fs" />

src/FSharp.Data.Xml.Core/FSharp.Data.Xml.Core.fsproj

@@ -9,7 +9,7 @@
     <PackageIcon>logo.png</PackageIcon>
     <!-- always have tailcalls on for design time compiler add-in to allow repo to compile in DEBUG, see https://github.com/fsprojects/FSharp.Data/issues/1410 -->
     <Tailcalls>true</Tailcalls>
-    <IsTrimmable>true</IsTrimmable>
+    <IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">true</IsTrimmable>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="XmlRuntime.fs" />