.Net: Fix postgres memory store dispose (#3177)

### Motivation and Context

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Fix #3121

### Description

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Convert to `IDisposable` to execute `Dispose`.

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---------

Co-authored-by: Roger Barreto <[email protected]>

dotnet/src/Connectors/Connectors.Memory.Postgres/PostgresMemoryStore.cs

@@ -210,7 +210,9 @@ protected virtual void Dispose(bool disposing)
     {
         if (disposing)
         {
-            this._dataSource?.Dispose();
+            // Avoid error when running in .Net 7 where it throws
+            // Could not load type 'System.Data.Common.DbDataSource' from assembly 'Npgsql, Version=7.*
+            (this._dataSource as IDisposable)?.Dispose();
         }
     }