Enable the Floating labels in forms

use field group and field group bootstrap → you will have floating label

Thanks everyone for contribution. 🍻
Obviously the issue can be solved without modifying the theme.
Closing...

I got floating labels in my subtheme with these steps:

1. Copy the input.html.twig file from /bootstrap5/templates/form to templates/form folder in my sub-theme
2. Tweak this part, from:
   <input{{ attributes }} />
   To:

   <div class="form-floating">
     <input{{ attributes }} />
     <label for="{{ attributes.id }}">{{ attributes.placeholder }}</label>
   </div>
   

I got the id's to match and placeholder, by setting them in my custom form:

    $form['age'] = [
      '#type' => 'number',
      '#title' => $this->t('Age'),
      '#placeholder' => 'Years old',
      '#attributes' => [
        'class' => ['age'],
        'id' => ['age'],
      ],
      '#required' => TRUE,
    ];