refactor(datasource): use joinUrlParts

[ikesyo]

Changes

Use joinUrlParts for URL construction in datasources.

Context

https://github.com/renovatebot/renovate/pull/16545#discussion_r928207641

Documentation (please check one with an [x])

• [ ] I have updated the documentation, or
• [x] No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• [x] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository