fix: mark binding "security_context" as purely computed [ENG-6249] (#158)

[ENG-6249](https://stacklet.atlassian.net/browse/ENG-6249)

### what

mark the "security_context" attribute in the binding resource as not
optional (while still computed), making it purely a computed value

### why

this prevents practitioners from setting a value in the config, which is
not
supported, since setting is done via
"security_context_wo"/"security_context_wo_version".
With this, terraform raises a proper error

### testing

local testing

### docs

n/a


[ENG-6249]:
https://stacklet.atlassian.net/browse/ENG-6249?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: albertodonato

internal/resources/binding.go

@@ -123,7 +123,6 @@ func (r *bindingResource) Schema(_ context.Context, _ resource.SchemaRequest, re
 			},
 			"security_context": schema.StringAttribute{
 				Description: "The binding execution security context.",
-				Optional:    true,
 				Computed:    true,
 			},
 			"security_context_wo": schema.StringAttribute{