Dispose LdapConnections used by ValidateCredentials

Ensure that cached LdapConnection instances created by
PrincipalContext.ValidateCredentials are disposed when
the corresponding PrincipalContext is disposed.

Fix dotnet#62035

src/libraries/System.DirectoryServices.AccountManagement/src/System/DirectoryServices/AccountManagement/Context.cs

@@ -44,7 +44,7 @@ internal static class CapabilityMap
         public const string LDAP_CAP_ACTIVE_DIRECTORY_V61_OID = "1.2.840.113556.1.4.1935";
     }
 
-    internal sealed class CredentialValidator
+    internal sealed class CredentialValidator : IDisposable
     {
         private enum AuthMethod
         {
@@ -341,6 +341,14 @@ public bool Validate(string userName, string password, ContextOptions connection
                 return (BindSam(_serverName, userName, password));
             }
         }
+
+        public void Dispose()
+        {
+            foreach (LdapConnection connection in _connCache.Values)
+            {
+                connection.Dispose();
+            }
+        }
     }
     // ********************************************
     public class PrincipalContext : IDisposable
@@ -1014,6 +1022,8 @@ public void Dispose()
                 if (_queryCtx != null)
                     _queryCtx.Dispose();
 
+                _credValidate.Dispose();
+
                 _disposed = true;
                 GC.SuppressFinalize(this);
             }