[FEAT] Restore factory method and optimize record implementation

- Added the static factory method `of(String username, String password)` to maintain compatibility with existing Selenium code that relies on it.
- Optimized the constructor by using a compact form, ensuring `username` and `password` are non-null without explicitly redefining fields.
- Removed redundant `username()` and `password()` methods, as they are automatically generated by the `record` structure.
- Improved code clarity, maintainability, and adherence to Java's record best practices.

Author: manuelsblanco

java/src/org/openqa/selenium/UsernameAndPassword.java

@@ -22,8 +22,12 @@
 /** A combination of username and password to use when authenticating a browser with a website. */
 public record UsernameAndPassword(String username, String password) implements Credentials {
 
-  public UsernameAndPassword(String username, String password) {
-    this.username = Require.nonNull("User name", username);
-    this.password = Require.nonNull("Password", password);
+  public UsernameAndPassword {
+    username = Require.nonNull("User name", username);
+    password = Require.nonNull("Password", password);
+  }
+
+  public static UsernameAndPassword of(String username, String password) {
+    return new UsernameAndPassword(username, password);
   }
 }