chore: misc go 1.27 tweaks (#39069)

Follow-up to https://github.com/go-gitea/gitea/pull/39068, which
disabled `modernize` entirely.

- re-enable `modernize`, with only the new `embedlit` rule disabled. It
flattens embedded struct literals across ~145 files, and orphans imports
in 6 of them that the fixer does not remove
- apply the rest of the suite: `errors.AsType`, `reflect.TypeAssert`,
`strings.Cut`, and dropping the legacy import comment
- use the new stdlib `uuid` package, `github.com/google/uuid` becomes
indirect
- use `strings.CutLast` in place of manual `LastIndex` slicing in label
scopes, email domains and the diff tree list
- take the header lint skip dirs from the `go.mod` `ignore` directive
and skip dot-directories, instead of hardcoding the list

Assisted-by: Claude Code:claude-opus-5

models/webhook/webhook_test.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"testing"
 	"time"
+	"uuid"
 
 	"gitea.dev/models/db"
 	"gitea.dev/models/unittest"
@@ -15,7 +16,6 @@ import (
 	"gitea.dev/modules/timeutil"
 	webhook_module "gitea.dev/modules/webhook"
 
-	"github.com/google/uuid"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"xorm.io/builder"