Update to .NET 6 and C# 10 (#5233)

Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Content.IntegrationTests/Tests/GameObjects/Components/Movement/ClimbUnitTest.cs

@@ -55,7 +55,7 @@ namespace Content.IntegrationTests.Tests.GameObjects.Components.Movement
                 // Test for climb components existing
                 // Players and tables should have these in their prototypes.
                 Assert.That(human.TryGetComponent(out climbing!), "Human has no climbing");
-                Assert.That(table.TryGetComponent(out ClimbableComponent _), "Table has no climbable");
+                Assert.That(table.TryGetComponent(out ClimbableComponent? _), "Table has no climbable");
 
                 // Now let's make the player enter a climbing transitioning state.
                 climbing.IsClimbing = true;