[BUG] html generator output has unescaped xml

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

HTML generated by the html generator has unescaped XML in the examples section.

Preview:

  123456789
  doggie
  
    aeiou
  
  
  
  aeiou

Code generated:

<pre class="example"><code><pet>
 <id>123456789</id>
 <name>doggie</name>
 <photourls>
   <photourls>aeiou</photourls>
 </photourls>
 <tags>
 </tags>
 <status>aeiou</status>
</pet></code></pre>

Code expected:

<pre class="example"><code><pet>
  <id>123456789</id>
  <name>doggie</name>
  <photourls>
    <photourls>aeiou</photourls>
  </photourls>
  <tags>
  </tags>
  <status>aeiou</status>
</pet></code></pre>

openapi-generator version

Latest master. The tool prints 6.2.1-SNAPSHOT

OpenAPI declaration file content or url

modules/openapi-generator/src/test/resources/3_0/petstore.yaml from the repo itself.

Steps to reproduce

java -jar ./modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i modules/openapi-generator/src/test/resources/3_0/petstore.yaml -g html -o /tmp/index.html

Related issues/PRs

Multiple issues related to HTML entities, but none related to unescaped XML, it seems.

Run the documented generate command with modules/openapi-generator/src/test/resources/3_0/petstore.yaml and inspect the html generator's examples section. Compare the generated output with the expected escaped markup; done means XML tags appear as HTML entities rather than rendered as XML.