Use arguments instead of argumentList in the macro template (#7424)

* **Explanation**: `argumentList` has been deprecated. The Swift macro
template should use `arguments` instead.
* **Scope**: The Swift macro template
* **Risk**: Very low
* **Testing**: Made the same modification in a macro package that was
freshly generated from the template and verified that it builds without
warnings
* **Issue**: n/a
* **Reviewer**: @MaxDesiatov on
#7422

Sources/Workspace/InitPackage.swift

@@ -836,7 +836,7 @@ public final class InitPackage {
  of node: some FreestandingMacroExpansionSyntax,
  in context: some MacroExpansionContext
  ) -> ExprSyntax {
- guard let argument = node.argumentList.first?.expression else {
+ guard let argument = node.arguments.first?.expression else {
  fatalError("compiler bug: the macro does not have any arguments")
  }