Use Golang net/smtp instead of gomail's smtp to send email (#36055)

Replace #36032
Fix #36030

This PR use `net/smtp` instead of gomail's smtp. Now
github.com/wneessen/go-mail will be used only for generating email
message body.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

services/mailer/sender/smtp_test.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import "testing"
+
+func TestSanitizeEmailAddress(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected string
+		hasError bool
+	}{
+		{"abc@gitea.com", "abc@gitea.com", false},
+		{"<abc@gitea.com>", "abc@gitea.com", false},
+		{"ssss.com", "", true},
+		{"<invalid-email>", "", true},
+	}
+
+	for _, tt := range tests {
+		result, err := sanitizeEmailAddress(tt.input)
+		if (err != nil) != tt.hasError {
+			t.Errorf("sanitizeEmailAddress(%q) unexpected error status: got %v, want error: %v", tt.input, err != nil, tt.hasError)
+			continue
+		}
+		if result != tt.expected {
+			t.Errorf("sanitizeEmailAddress(%q) = %q; want %q", tt.input, result, tt.expected)
+		}
+	}
+}