Draft: Removes all the tech debt surrounding our unified diff lines

What does this MR do?

This removes all the tech debt added with our unified diff lines feature flag. With the flag enabled we would always be accessing parallel_diff_lines so this removes all the code that access highlighted_diff_lines. Whilst we are here, it also changes the code to be using a constant which allows us to change the name of the key after this.

Whilst I was here I also changed all the code that touches .left or .right to use a constant as well so that we can move away from left & right to something more meaningful.

Closes #241188 (closed)

Screenshots

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