Fix: Code block input rule doesn’t save the code block language

What does this MR do?

This MR fixes a bug that didn’t store a code block language when inserting a code block using an Input Rule. The solution for this problem is rewriting the Markdown Serializer for code blocks to use a language attribute instead of a params attribute.

The code block extension sets the language attribute by default when inserting a code block using an input rule.

Screenshots or Screencasts (strongly suggested)

Inserting a code block	Viewing the code block

How to setup and validate locally (strongly suggested)

• Edit a Wiki page in your local environment
• Choose the Content Editor as your editor of choice
• Insert a code block typing the following keys: ```javascript(space)
• The Content Editor will create a code block that highlights javascript code.
• Submit changes
• When viewing the Wiki page, the code block should still highlight JavaScript code.

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team