enhance: Migrate remaining gopkg.in/yaml.v3 usages to go.yaml.in/yaml/v4 (#37866)

### 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>

modules/structs/issue.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 	"time"
 
-	"gopkg.in/yaml.v3"
+	"go.yaml.in/yaml/v4"
 )
 
 // StateType issue state type
@@ -231,7 +231,7 @@ func (l *IssueTemplateStringSlice) UnmarshalYAML(value *yaml.Node) error {
 		*l = labels
 		return nil
 	}
-	return fmt.Errorf("line %d: cannot unmarshal %s into IssueTemplateStringSlice", value.Line, value.ShortTag())
+	return fmt.Errorf("cannot unmarshal %s into IssueTemplateStringSlice", value.ShortTag())
 }
 
 type IssueConfigContactLink struct {

modules/structs/issue_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"gopkg.in/yaml.v3"
+	"go.yaml.in/yaml/v4"
 )
 
 func TestIssueTemplate_Type(t *testing.T) {
@@ -88,7 +88,7 @@ labels:
   b: bb
 `,
 			tmpl:    &IssueTemplate{},
-			wantErr: "line 3: cannot unmarshal !!map into IssueTemplateStringSlice",
+			wantErr: "yaml: unmarshal errors:\n  line 3: cannot unmarshal !!map into IssueTemplateStringSlice",
 		},
 	}
 	for _, tt := range tests {