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

cmd/headscale/cli/serve.go

@@ -23,8 +23,7 @@ var serveCmd = &cobra.Command{
 	Run: func(cmd *cobra.Command, args []string) {
 		app, err := newHeadscaleServerWithConfig()
 		if err != nil {
-			var squibbleErr squibble.ValidationError
-			if errors.As(err, &squibbleErr) {
+			if squibbleErr, ok := errors.AsType[squibble.ValidationError](err); ok {
 				fmt.Printf("SQLite schema failed to validate:\n")
 				fmt.Println(squibbleErr.Diff)
 			}

cmd/hi/run.go

@@ -76,7 +76,7 @@ func detectGoVersion() string {
 
 	content, err := os.ReadFile(goModPath)
 	if err != nil {
-		return "1.25"
+		return "1.26rc2"
 	}
 
 	lines := splitLines(string(content))
@@ -91,7 +91,7 @@ func detectGoVersion() string {
 		}
 	}
 
-	return "1.25"
+	return "1.26rc2"
 }
 
 // splitLines splits a string into lines without using strings.Split.