[Form] Deprecated bind() and isBound() in favor of submit() and isSubmitted()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #5493
| License       | MIT
| Doc PR        | TODO

This change was discussed for a while in #5493. It breaks BC *only for people who implemented* `FormInterface` *manually* (not a lot, so I hope). These can fix the problem by simply renaming `bind()` and `isBound()` in their implementation to `submit()` and `isSubmitted()`.

The main rationale is that with the request handlers (form processors) introduced in #6522, people won't be confronted with the term "binding" anymore. As such, `isBound()` will be a very strange name to new users that have never used `bind()` manually.

See this code sample as example:

```php
$form = $this->createForm(...);

$form->handleRequest($request);

// Imagine you have never heard about bind() or binding. What does this mean?
if ($form->isBound()) {
    // ...
}
```

In reality, `bind()` submits a form. Where-ever I renamed "bind" to "submit" in the comments, "submit" made actually much more sense. So it does in the code sample above:

```php
$form = $this->createForm(...);

$form->handleRequest($request);

// Aha!
if ($form->isSubmitted()) {
    // ...
}
```

Also when using `submit()` directly, the code makes much more sense now:

```php
$text = $this->createForm('text');
$text->submit('New Value');
```