C++: Add change note

Author: jketema

cpp/ql/lib/change-notes/2024-12-26-requires-clause.md

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+---
+category: feature
+---
+* New predicates `getARequiresClause`, `getTemplateRequiresClause` and `getFunctionRequiresClause` were added to the `FunctionDeclarationEntry` class, which yield the requires clauses when the entry represents a function template declaration with requires clauses.
+* A new predicate `getRequiresClause` was added to the `TypeDeclarationEntry` class, which yields the requires clause when the entry represents a class template declaration with a requires clause.
+* A new predicate `getRequiresClause` was added to the `VariableDeclarationEntry` class, which yields the requires clause when the entry represents a variable template declaration with a requires clause.
+* A new predicate `getTypeConstraint` was added to the `TypeTemplateParameter` class, which yields the type constraint of the parameter if it exists.