JiraService: simplify url generation

What does this MR do?

• It simplifies the build_entity_url method by providing Settings.gitlab.base_url as a parameter of polymorphic_url method instead of using resource_url method additionally.
• resource_url method was kept as it is used by other methods in this class.

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

• base_url that was added by resource_url method before and now is provided as param of polymorphic_url is tested - I changed the base_url setting in one test and made sure tests are passing

Why was this MR needed?

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?

It fixes #24085 (closed)