Fix an apartment state issue (partial port of dotnet#19384)

Partial port of dotnet#19384 to 2.2

Fix for https://github.com/dotnet/coreclr/issues/17822
- The apartment state now defaults to MTA for the main thread along with a CoInitialize
- Calling `Thread.SetApartmentState` with STA now fails as expected (different behavior from previous netcore, same behavior as netfx)

Fix for https://github.com/dotnet/coreclr/issues/19225

src/vm/appdomain.cpp

@@ -3078,8 +3078,9 @@ void SystemDomain::SetThreadAptState (Thread::ApartmentState state)
         Thread::ApartmentState pState = pThread->SetApartment(Thread::AS_InSTA, TRUE);
         _ASSERTE(pState == Thread::AS_InSTA);
     }
-    else if (state == Thread::AS_InMTA)
+    else
     {
+        // If an apartment state was not explicitly requested, default to MTA
         Thread::ApartmentState pState = pThread->SetApartment(Thread::AS_InMTA, TRUE);
         _ASSERTE(pState == Thread::AS_InMTA);
     }

src/vm/assembly.cpp

@@ -1798,11 +1798,7 @@ INT32 Assembly::ExecuteMainMethod(PTRARRAYREF *stringArgs, BOOL waitForOtherThre
 
                 Thread::ApartmentState state = Thread::AS_Unknown;
                 state = SystemDomain::GetEntryPointThreadAptState(pMeth->GetMDImport(), pMeth->GetMemberDef());
-
-                // If the entry point has an explicit thread apartment state, set it
-                // before running the AppDomainManager initialization code.
-                if (state == Thread::AS_InSTA || state == Thread::AS_InMTA)
-                    SystemDomain::SetThreadAptState(state);
+                SystemDomain::SetThreadAptState(state);
 #endif // FEATURE_COMINTEROP
             }