Refactor collection initializers to use C# 12 collection expressions

This commit updates various files across the Core and WebApp projects to replace traditional collection initializers with C# 12 collection expressions. Changes include modifications to EventAssignment.cs, TeamScheduler_DecisionTree.cs, CareerField.cs, EventDefinition.cs, and several components in the WebApp. These updates enhance code readability and maintainability by adhering to modern C# syntax standards.

Core/Parsers/StudentParser.cs

@@ -13,7 +13,7 @@ public class StudentParser : CsvParserBase
 
     public Student[] Parse()
     {
-        var s = new List<Student>();
+        var students = new List<Student>();
 
         CsvReader.Read();
         CsvReader.ReadHeader();
@@ -44,9 +44,9 @@ public class StudentParser : CsvParserBase
                 RegionalId = regionalId,
                 NationalId = nationalId
             };
-            s.Add(student);
+            students.Add(student);
 		}
 
-        return s.ToArray();
+        return students.ToArray();
     }
 }