Fix form submission handler regression introduced in class-to-functional conversion

This fixes a regression introduced in commit d12fad6
'Add support for visual branding of identifier' where the Login component was
converted from class-based to functional component.

During the conversion:
- The 'logon' method was correctly renamed to 'handleNextClick'
- The button's onClick was correctly updated to use 'handleNextClick'
- However, the form's onSubmit was left with the old 'this.logon(event)' reference

This resulted in a broken form submission handler that referenced a non-existent
method. The application continued to work because Enter key presses trigger the
submit button's onClick handler due to standard HTML form behavior, masking
the regression.

Changes:
- Replace broken 'onSubmit={(event) => this.logon(event)}' with 'onSubmit={handleNextClick}'
- Ensures consistent behavior between form submission and button click
- Maintains existing preventDefault behavior and proper form handling

Regression introduced: d12fad6 (Oct 12, 2021)
Component: identifier/src/containers/Login/Login.jsx:137

Author: longsleep

identifier/src/containers/Login/Login.jsx

@@ -134,7 +134,7 @@ function Login(props) {
         {t("konnect.login.headline", "Sign in")}
       </Typography>
 
-      <form action="" onSubmit={(event) => this.logon(event)}>
+      <form action="" onSubmit={handleNextClick}>
         <TextField
           label={usernamePlaceHolder}
           error={!!errors.username}