feat(plugin-form-builder): change checkbox label from 'Default Value' to 'Checked by default' (#15229)

### What?
Changed the default value label text in form builder for type checkbox.

### Why?
The old label text was confusing as in combination with a checkbox users
think they can toggle if they want a defaultValue or not. It's now
called "Checked by default" to make clear users can check if the
checkbox should be checked by default.

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: MariusMatu

packages/plugin-form-builder/src/collections/Forms/fields.ts

@@ -478,7 +478,7 @@ const Checkbox: Block = {
     {
       name: 'defaultValue',
       type: 'checkbox',
-      label: 'Default Value',
+      label: 'Checked by default',
     },
   ],
   labels: {