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

integration/auth_key_test.go

@@ -17,7 +17,6 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"tailscale.com/tailcfg"
-	"tailscale.com/types/ptr"
 )
 
 func TestAuthKeyLogoutAndReloginSameUser(t *testing.T) {
@@ -634,7 +633,7 @@ func TestAuthKeyLogoutAndReloginRoutesPreserved(t *testing.T) {
 				},
 				AutoApprovers: policyv2.AutoApproverPolicy{
 					Routes: map[netip.Prefix]policyv2.AutoApprovers{
-						netip.MustParsePrefix(advertiseRoute): {ptr.To(policyv2.Username(user + "@test.no"))},
+						netip.MustParsePrefix(advertiseRoute): {new(policyv2.Username(user + "@test.no"))},
 					},
 				},
 			},