[PL] Out; Deprecate sec. vars. type attribute.

Update docu and make the attribute optional.
Issue a warning in case it is set.

Documentation/ProjectFile/prj/processes/process/secondary_variables/secondary_variable/a_type.md

-The type of the secondary variable.
-
-Can be <tt>static</tt> for secondary variables that don't need any further
-information from the project file in order to be computed;
-or <tt>dynamic</tt> if the functions providing some of the input data for
-computing the secondary variable are set up at runtime.
+Deprecated. This attribute is no longer used.

ProcessLib/Output/CreateSecondaryVariables.cpp

@@ -37,7 +37,13 @@ void createSecondaryVariables(
     {
         auto const type =
             //! \ogs_file_attr{prj__processes__process__secondary_variables__secondary_variable__type}
-            sec_var_config.getConfigAttribute<std::string>("type");
+            sec_var_config.getConfigAttributeOptional<std::string>("type");
+        if (type)
+        {
+            WARN(
+                "Secondary variable type specification is deprecated and is "
+                "ignored. All secondary variable types are 'static'.");
+        }
 
         auto const internal_name =
             //! \ogs_file_attr{prj__processes__process__secondary_variables__secondary_variable__internal_name}