Fix editing of Markdown fields when cache version is incremented

What does this MR do?

This fixes the cache invalidation logic for Markdown fields

This removes the extra optimization where we skip rendering when we see that both the markdown field and the HTML field are both updated.

This optimization is unreliable because even if both fields are changed, we cannot be sure that the HTML is correct for the current markdown value.

#208255 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] 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