**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


tests/www/test_utils.py
rendered = wrapped_markdown("*italic*\n**bold**\n", css_class="a_class")
        assert (
            '''<div class="a_class" ><p><em>italic</em>
<strong>bold</strong></p></div>'''
            == rendered
        )
    def test_wrapped_markdown_with_table(self):
        rendered = wrapped_markdown(
            """| Job | Duration |
               | ----------- | ----------- |
               | ETL | 14m |"""
        )
        assert (
            '<div class="rich_doc" ><table>\n<thead>\n<tr>\n<th>Job</th>\n'
            '<th>Duration</th>\n</tr>\n</thead>\n<tbody>\n<tr>\n<td>ETL'
            '</td>\n<td>14m</td>\n</tr>\n</tbody>\n'
            '</table></div>'
        ) == rendered
    def test_wrapped_markdown_with_indented_lines(self):
        rendered = wrapped_markdown(
            """
                # header
                1st line
                2nd line
            """
        )
        assert '<div class="rich_doc" ><h1>header</h1>\n<p>1st line\n2nd line</p></div>' == rendered