### Description

https://github.com/apache/airflow/pull/17946 implements a `/robots.txt` endpoint to block search engines crawling Airflow - in the cases where it is (accidentally) exposed to the public Internet.

If we record any GET requests to that end-point we'd have a strong warning flag that the deployment is exposed, and could issue a warning in the UI, or even enable some kill-switch on the deployment. 

Some deployments are likely intentionally available and rely on auth mechanisms on the `login` endpoint, so there should be a config option to suppress the warnings.

An alternative approach would be to monitor for requests from specific user-agents used by crawlers for the same reasons

### Use case/motivation

People who accidentally expose airflow have a slightly higher chance of realising they've done so and tighten their security.

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] 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)

airflow/www/views.py
"""Formats hidden fields"""
        key = self.get('key')
        val = self.get('val')
        if secrets_masker.should_hide_value_for_key(key):
            return Markup('*' * 8)
        if val:
            return val
        else:
            return Markup('<span class="label label-danger">Invalid</span>')
    formatters_columns = {
        'val': hidden_field_formatter,
    }
    validators_columns = {'key': [validators.DataRequired()]}
    def prefill_form(self, form, request_id):
        if secrets_masker.should_hide_value_for_key(form.key.data):
            form.val.data = '*' * 8
    extra_args = {"can_create_variable": _can_create_variable}
    @action('muldelete', 'Delete', 'Are you sure you want to delete selected records?', single=False)
    def action_muldelete(self, items):
        """Multiple delete."""
        self.datamodel.delete_all(items)
        self.update_redirect()
        return redirect(self.get_redirect())
    @action('varexport', 'Export', '', single=False)
    def action_varexport(self, items):