**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
def setUp(self):
        self.attr_renderer = utils.get_attr_renderer()
    def test_python_callable(self):
        def example_callable(unused_self):
            print("example")
        rendered = self.attr_renderer["python_callable"](example_callable)
        assert '&quot;example&quot;' in rendered
    def test_python_callable_none(self):
        rendered = self.attr_renderer["python_callable"](None)
        assert "" == rendered
    def test_markdown(self):
        markdown = "* foo\n* bar"
        rendered = self.attr_renderer["doc_md"](markdown)
        assert "<li>foo</li>" in rendered
        assert "<li>bar</li>" in rendered
    def test_markdown_none(self):
        rendered = self.attr_renderer["python_callable"](None)
        assert "" == rendered
class TestWrappedMarkdown(unittest.TestCase):
    def test_wrapped_markdown_with_docstring_curly_braces(self):
        rendered = wrapped_markdown("{braces}", css_class="a_class")
        assert '<div class="a_class" ><p>{braces}</p></div>' == rendered
    def test_wrapped_markdown_with_some_markdown(self):