Use iids as filter parameter

What does this MR do?

iids is used as parameter for filtering milestones by iid instead of iid so that we're consistent across the API.

Why was this MR needed?

API consistency

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?

Fixes #27532 (closed)

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• 1d56fb08 - Use iids as filter parameter

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I think this is missing a change in the API documentation and a change in the V3 vs. V4 api documentation. Furthermore, this needs to be backported to the V3 API.

added ~13884 ~728081 labels

Yep, I forgot for the documentation, will update it. Thanks

assigned to @jarka

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• 49d7085e - Use iids as filter parameter

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• 66d369c7 - Use iids as filter parameter

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I added the documentation and added milestones to v3 endpoints. I hope it is ready to review

assigned to @vsizov

added 263 commits

• 66d369c7...edb8ed36 - 262 commits from branch gitlab-org:master
• fdff9f81 - Use iids as filter parameter

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@jarka please pick another reviewer, Valery is out this week

assigned to @rdavila

@jarka thanks for working on it, I just left a small comment, other than that it LGTM!

assigned to @jarka

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• e5174439 - Use iids as filter parameter

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Thanks for the feedback @rdavila . I added specs for case no milestone is found and iid is invalid

assigned to @rdavila

added 1154 commits

• e5174439...90040620 - 1153 commits from branch gitlab-org:master
• 0f3de3ce - Use iids as filter parameter

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