Language of code block manage should be changed to lowercase

[seonim-ryu]

Describe the bug

When the language of the code block contains uppercase letters, the contents of the code block are not rendered normally.

To Reproduce

Steps to reproduce the behavior:

1. In Markdown, enter the fenced code block.
2. Enter a mix of uppercase and lowercase letters in the language of the code block.

\`\`\`PlantUML
\`\`\`

or

\`\`\`PlantUml
\`\`\`

Expected behavior

It should work as if entering a lowercase letter.

\`\`\`plantuml
\`\`\`