**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
import warnings
    from airflow.utils.log.secrets_masker import get_sensitive_variables_fields
    warnings.warn(
        "This function is deprecated. Please use "
        "`airflow.utils.log.secrets_masker.get_sensitive_variables_fields`",
        DeprecationWarning,
        stacklevel=2,
    )
    return get_sensitive_variables_fields()
def should_hide_value_for_key(key_name):  
    import warnings
    from airflow.utils.log.secrets_masker import should_hide_value_for_key
    warnings.warn(
        "This function is deprecated. Please use "
        "`airflow.utils.log.secrets_masker.should_hide_value_for_key`",
        DeprecationWarning,
        stacklevel=2,
    )
    return should_hide_value_for_key(key_name)
def get_params(**kwargs):