Remove unsupported LangVersion from dotnet example (#2003)

dotnet builds started failing, due to the upgrade of the github runners
to ubuntu 24.04.
The `RoleCS` test was failing with issues in selecting `LanguageVersion`
11.

I've removed that version identifier from the `csproj` of the failing
test to align with the other CS tests and changed it to not use features
from language version 11.

### Why did this work before?
Previously this worked, because the test implicitly used v8 of the
dotnet runtime instead of the requested v6 (configured in ci-mgmt
pulumi/ci-mgmt#1307).

The `setup-dotnet` action implicitly installs the latest LTS before
installing other versions.
We're requesting 6.0.x and on Ubuntu 22.04 this was already
pre-installed, so this happened:
- Install latest LTS (v8)
- Install v6.0.x -> Already present -> Noop

Because of that v8 was on the path first and used for tests.

Now on Ubuntu 24.04 v8 is pre-installed, so this happens:
- Install latest LTS (v8) -> Already present -> Noop
- Install v6.0.x

Now we're actually using the requested v6, but the `RoleCs` test is not
compatible with that.

examples/role-cs/MyStack.cs

@@ -12,19 +12,17 @@ public MyStack()
     {
         var testRole = new Role("role-test", new RoleArgs
         {
-            AssumeRolePolicyDocument = """
-            {
-                "Version": "2012-10-17",
-                "Statement": [{
-                    "Sid": "",
-                    "Effect": "Allow",
-                    "Principal": {
-                        "Service": "ec2.amazonaws.com"
+            AssumeRolePolicyDocument = @"{
+                ""Version"": ""2012-10-17"",
+                ""Statement"": [{
+                    ""Sid"": """",
+                    ""Effect"": ""Allow"",
+                    ""Principal"": {
+                        ""Service"": ""ec2.amazonaws.com""
                     },
-                    "Action": "sts:AssumeRole"
+                    ""Action"": ""sts:AssumeRole""
                 }]
-            }
-            """,
+            }",
             Policies = new[]
             {
                 new IamInputs.RolePolicyArgs

examples/role-cs/role-cs.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <LangVersion>11</LangVersion>
     <OutputType>Exe</OutputType>
     <TargetFramework>net6.0</TargetFramework>
     <Nullable>enable</Nullable>