Rehighlights dcc.Markdown when it is updated

[leeagustin]

Fixes #2895.

The problem is that highlight.js was changed in highlightjs/highlight.js@3987abe, which prevents re-highlighting an element.

The attribute data-highlighted="yes" flags an element as highlighted. Hence, the solution is to remove this attribute before the code is re-highlighted.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Re-highlights dcc.Markdown after it is updated
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[gvwilson]

@T4rk1n can you please have a look and let us know if we can merge?

[T4rk1n]

💃 Looks good, thank you.