Drop net7.0 target (#1468)

* Drop net7.0 target

.NET 7 is EOL since May. The only .NET 7 features we use are
`ObjectDisposedException.ThrowIf` (moved to a throw helper) and
some newer regex features.

This feels a bit weird, but I suppose it is the expected course of action.

* fix build warning-as-error which is suddenly appearing on net6.0

IsAotCompatible not supported on net6.0

---------

Co-authored-by: Wojciech Nagórski <wojtpl2@gmail.com>

src/Renci.SshNet/Renci.SshNet.csproj

@@ -4,7 +4,7 @@
     <AssemblyName>Renci.SshNet</AssemblyName>
     <Product>SSH.NET</Product>
     <AssemblyTitle>SSH.NET</AssemblyTitle>
-    <TargetFrameworks>net462;netstandard2.0;netstandard2.1;net6.0;net7.0;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>net462;netstandard2.0;netstandard2.1;net6.0;net8.0;net9.0</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup>
@@ -37,6 +37,10 @@
   </PropertyGroup>
 
   <PropertyGroup Condition=" $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0')) ">
+    <IsTrimmable>true</IsTrimmable>
+  </PropertyGroup>
+  
+  <PropertyGroup Condition=" $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0')) ">
     <IsAotCompatible>true</IsAotCompatible>
   </PropertyGroup>
 

test/Renci.SshNet.Tests/Renci.SshNet.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net462;net6.0;net7.0;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>net462;net6.0;net8.0;net9.0</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>