### Apache Airflow version

2.5.0

### What happened

https://github.com/apache/airflow/pull/20888 enables the use of markup to style the `webserver.instance_name`.
However, if the instance name has HTML code, this will also be reflected in the `<title>` tag, as shown in the screenshot below.
![image](https://user-images.githubusercontent.com/562969/212091882-d33bb0f7-75c2-4c92-bd4f-4bc7ba6be8db.png)

This is not a pretty behaviour.

### What you think should happen instead

Ideally, if `webserver. instance_name_has_markup = True`, then the text inside the `<title>` should be stripped of HTML code.

For example:
- Set `webserver.instance_name` to some text with markup, like `<b style="color: red">title</b>`
- Set `webserver.Instance_name_has_markup` to `true`

This is how the `<title>` tag should look like:

```html
<title>DAGs - title</title>
```

Instead of:

```
<title>DAGs - &lt;b style=&#34;color: red&#34;&gt;title&lt;b&gt;</title>
```

### How to reproduce

- Airflow version 2.3+, which is [when this change has been introduced](https://airflow.apache.org/docs/apache-airflow/stable/configurations-ref.html#instance-name-has-markup)
- Set `webserver.instance_name` to some text with markup, like `<b style="color: red">title</b>`
- Set `webserver.Instance_name_has_markup` to `true`


### Operating System

Doesn't matter

### Versions of Apache Airflow Providers

_No response_

### Deployment

Other

### Deployment details

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

resp = viewer_client.get(url, follow_redirects=True)
    check_content_in_response("Access is Denied", resp)
@pytest.fixture()
def non_exist_username(app):
    username = "fake_username"
    user = app.appbuilder.sm.find_user(username)
    if user is not None:
        app.appbuilder.sm.del_register_user(user)
    yield username
    user = app.appbuilder.sm.find_user(username)
    if user is not None:
        app.appbuilder.sm.del_register_user(user)
def test_create_user(app, admin_client, non_exist_username):
    resp = admin_client.post(
        "users/add",
        data={
            "first_name": "fake_first_name",
            "last_name": "fake_last_name",
            "username": non_exist_username,
            "email": "fake_email@email.com",
            "roles": [1],
            "password": "test",
            "conf_password": "test",
        },
        follow_redirects=True,
    )
    check_content_in_response("Added Row", resp)
    assert app.appbuilder.sm.find_user(non_exist_username)
@pytest.fixture()
def exist_username(app, exist_role):