fix Euler acoustics callback error (#1244)

Caused by an update of SciMLBase.jl.

src/callbacks_step/euler_acoustics_coupling.jl

@@ -176,7 +176,11 @@ function (euler_acoustics_coupling::EulerAcousticsCouplingCallback)(integrator_a
   if !isfinished(integrator_euler)
     @trixi_timeit timer() "Euler solver" step!(integrator_euler)
     return_code = check_error(integrator_euler)
-    if return_code !== :Success && return_code !== :Default
+    # Check the return code with `!=` instead of `!==` recommended for
+    # `Symbol`s in general since the return code can be a plain symbol 
+    # or an enum overloading `==` and `!=`, depending on the version of
+    # SciMLBase.jl.
+    if return_code != :Success && return_code != :Default
       error("Error during compressible Euler time integration. Received return code $(return_code)")
     end
   end
@@ -202,4 +206,4 @@ end
 
 include("euler_acoustics_coupling_dg2d.jl")
 
-end # @muladd
+end # @muladd