Fix code highlight: remove 'markdown' from automatic language detection.

[isoos]

• Fixes Underscores should not italicize inside of a code block in Markdown #8861.
• Verified locally: highlight.js will use markdown language is the code block is tagged with it, but won't use it in automatic language detection, when there is no language tag on the code block.
• Note: I was also looking for a way to define these two list from a single source, but (1) the list is short, (2) in a single place, (3) rarely updated, so let's just keep it via copy-paste.
• The configuration being used: https://highlightjs.readthedocs.io/en/latest/api.html#configure