### 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)

flask_app.config["SESSION_COOKIE_HTTPONLY"] = True
    flask_app.config["SESSION_COOKIE_SECURE"] = conf.getboolean("webserver", "COOKIE_SECURE")
    cookie_samesite_config = conf.get("webserver", "COOKIE_SAMESITE")
    if cookie_samesite_config == "":
        warnings.warn(
            "Old deprecated value found for `cookie_samesite` option in `[webserver]` section. "
            "Using `Lax` instead. Change the value to `Lax` in airflow.cfg to remove this warning.",
            RemovedInAirflow3Warning,
        )
        cookie_samesite_config = "Lax"
    flask_app.config["SESSION_COOKIE_SAMESITE"] = cookie_samesite_config
    if config:
        flask_app.config.from_mapping(config)
    if "SQLALCHEMY_ENGINE_OPTIONS" not in flask_app.config:
        flask_app.config["SQLALCHEMY_ENGINE_OPTIONS"] = settings.prepare_engine_args()
    
    flask_app.json_provider_class = AirflowJsonProvider
    flask_app.json = AirflowJsonProvider(flask_app)
    InternalApiConfig.force_database_direct_access()
    csrf.init_app(flask_app)
    init_wsgi_middleware(flask_app)
    db = SQLA()
    db.session = settings.Session
    db.init_app(flask_app)
    init_dagbag(flask_app)
    init_api_experimental_auth(flask_app)
    init_robots(flask_app)
    cache_config = {"CACHE_TYPE": "flask_caching.backends.filesystem", "CACHE_DIR": gettempdir()}
    Cache(app=flask_app, config=cache_config)
    init_flash_views(flask_app)