MDL-77328 qtype_calculated: Add upgrade.txt notes

* Question type plugins that extend qtype_calculated should be made
aware of the changes about the new question_answer type
\qtype_calculated\qtype_calculated_answer.

question/type/calculated/upgrade.txt

@@ -0,0 +1,6 @@
+This file describes API changes in /question/type/calculated/*.
+
+=== 4.2.2 ===
+* \qtype_calculated answers are now of the type \qtype_calculated\qtype_calculated_answer which extends \qtype_numerical_answer.
+  Plugins extending \qtype_calculated and still reference \qtype_numerical_answer in their code (e.g. in test helper functions)
+  should update their code to reference \qtype_calculated\qtype_calculated_answer instead.