**Apache Airflow version**: 2.0.0 - 2.1.0

**Kubernetes version**: N/A

**Environment**:

- **Cloud provider or hardware configuration**: Docker on MacOS (but also AWS ECS deployed)
- **OS** (e.g. from /etc/os-release): MacOS Big Sur 11.3.1
- **Kernel** (e.g. `uname -a`): Darwin Kernel Version 20.4.0
- **Install tools**:
- **Others**:

**What happened**:

When a code block is a part of a DAG's `doc_md`, it does not render correctly in the Web UI, but collapses all the lines into one line instead.

**What you expected to happen**:

The multi line code block be rendered with line breaks preserved.

**How to reproduce it**:
Create a DAG with `doc_md` containing a code block:

````python
from airflow import DAG

DOC_MD = """\
# Markdown code block

Inline `code` works well.

```
Code block
does not
respect
newlines
```
"""

dag = DAG(
    dag_id='test',
    doc_md=DOC_MD
)
````

The rendered documentation looks like this:
<img src="https://user-images.githubusercontent.com/11132999/119981579-19a70600-bfbe-11eb-8036-7d981ae1f232.png" width="50%"/>

**Anything else we need to know**: N/A


airflow/www/utils.py
"""
    FAB does not know how to handle columns with leading underscores because
    they are not supported by WTForm. This hack will remove the leading
    '_' from the key to lookup the column names.
    """
    def __init__(self, obj, session=None):
        super().__init__(obj, session=session)
        def clean_column_names():
            if self.list_properties:
                self.list_properties = {k.lstrip('_'): v for k, v in self.list_properties.items()}
            if self.list_columns:
                self.list_columns = {k.lstrip('_'): v for k, v in self.list_columns.items()}
        clean_column_names()
    def is_utcdatetime(self, col_name):
        """Check if the datetime is a UTC one."""
        from airflow.utils.sqlalchemy import UtcDateTime
        if col_name in self.list_columns:
            obj = self.list_columns[col_name].type
            return (
                isinstance(obj, UtcDateTime)
                or isinstance(obj, sqla.types.TypeDecorator)
                and isinstance(obj.impl, UtcDateTime)
            )
        return False
    filter_converter_class = UtcAwareFilterConverter
FieldConverter.conversion_table = (
    ('is_utcdatetime', DateTimeWithTimezoneField, AirflowDateTimePickerWidget),
) + FieldConverter.conversion_table