all: upgrade to Go 1.26rc2 and modernize codebase

This commit upgrades the codebase from Go 1.25.5 to Go 1.26rc2 and
adopts new language features.

Toolchain updates:
- go.mod: go 1.25.5 → go 1.26rc2
- flake.nix: buildGo125Module → buildGo126Module, go_1_25 → go_1_26
- flake.nix: build golangci-lint from source with Go 1.26
- Dockerfile.integration: golang:1.25-trixie → golang:1.26rc2-trixie
- Dockerfile.tailscale-HEAD: golang:1.25-alpine → golang:1.26rc2-alpine
- Dockerfile.derper: golang:alpine → golang:1.26rc2-alpine
- .goreleaser.yml: go mod tidy -compat=1.25 → -compat=1.26
- cmd/hi/run.go: fallback Go version 1.25 → 1.26rc2
- .pre-commit-config.yaml: simplify golangci-lint hook entry

Code modernization using Go 1.26 features:
- Replace tsaddr.SortPrefixes with slices.SortFunc + netip.Prefix.Compare
- Replace ptr.To(x) with new(x) syntax
- Replace errors.As with errors.AsType[T]

Lint rule updates:
- Add forbidigo rules to prevent regression to old patterns

hscontrol/db/preauth_keys_test.go

@@ -11,7 +11,6 @@ import (
 	"github.com/juanfont/headscale/hscontrol/util"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
-	"tailscale.com/types/ptr"
 )
 
 func TestCreatePreAuthKey(t *testing.T) {
@@ -24,7 +23,7 @@ func TestCreatePreAuthKey(t *testing.T) {
 			test: func(t *testing.T, db *HSDatabase) {
 				t.Helper()
 
-				_, err := db.CreatePreAuthKey(ptr.To(types.UserID(12345)), true, false, nil, nil)
+				_, err := db.CreatePreAuthKey(new(types.UserID(12345)), true, false, nil, nil)
 				assert.Error(t, err)
 			},
 		},
@@ -127,7 +126,7 @@ func TestCannotDeleteAssignedPreAuthKey(t *testing.T) {
 		Hostname:       "testest",
 		UserID:         &user.ID,
 		RegisterMethod: util.RegisterMethodAuthKey,
-		AuthKeyID:      ptr.To(key.ID),
+		AuthKeyID:      new(key.ID),
 	}
 	db.DB.Save(&node)