Remove .net framework 3.5 & 4.0 support

Author: scorteanucosmin

src/Oxide.Core.csproj

@@ -12,7 +12,7 @@
     <PackageIconUrl>icon.png</PackageIconUrl>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <PackageProjectUrl>$(RepositoryUrl)</PackageProjectUrl>
-    <TargetFrameworks>net35;net40;net45;net46;net48;netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>net45;net46;net48;netstandard2.0;netstandard2.1</TargetFrameworks>
     <LangVersion>Latest</LangVersion>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <NoWarn>NU1701</NoWarn>