Backport https://github.com/go-gitea/gitea/pull/39869
`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.
The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.
Only the milestone fix is backported. The issue and pull request
deadline changes of https://github.com/go-gitea/gitea/pull/39869 build
on https://github.com/go-gitea/gitea/pull/38677, which isn't on v28.
### Description
Replaces all remaining direct `gopkg.in/yaml.v3` imports with
`go.yaml.in/yaml/v4` across models, modules, routers, services, and
integration tests. `gopkg.in/yaml.v3` moves from a direct to an indirect
dependency in `go.mod`.
#### API compatibility
The yaml.Node type, node.Kind/node.Content traversal style
(modules/markup/markdown/convertyaml.go), and the
UnmarshalYAML(*yaml.Node) interface signature
(modules/optional/serialization.go) are all preserved in v4 — no
call-site changes were required beyond the import path.
**Related:**
- https://github.com/go-gitea/gitea/pull/36564#issuecomment-4526536805
---------
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>
