Use normal associations instead of polymorphic

What does this MR do?

Change the issuable association from polymorphic to regular associations.

Are there points in the code the reviewer needs to double check?

Yes, migrations.

Why was this MR needed?

Because we need to use foreign keys in order to have data integrity and improve the performance of the queries, this change makes sense given the timelogs table is going to be huge.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #26908 (closed)