chore: modernize the codebase using go 1.26 (#278)

This runs:

```
go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix ./...
```

over the codebase, as this is using go 1.26, it can also use the new
new() functionallity so AsPtr and boolPtr is are needed anymore.

Signed-off-by: Miek Gieben <miek@miek.nl>

internal/machine/api/proxy/director.go

@@ -107,7 +107,7 @@ func (d *Director) remoteBackend(addr string) (*RemoteBackend, error) {
 
 // FlushRemoteBackends closes all remote backend connections and removes them from the cache.
 func (d *Director) FlushRemoteBackends() {
-	d.remoteBackends.Range(func(key, value interface{}) bool {
+	d.remoteBackends.Range(func(key, value any) bool {
 		backend, ok := value.(*RemoteBackend)
 		if !ok {
 			return true

internal/machine/caddyconfig/caddyfile.go

@@ -226,12 +226,13 @@ func (g *CaddyfileGenerator) Generate(
 
 	// Append error summary as comment if there were any invalid configs.
 	if len(configErrors) > 0 {
-		errorsComment := "# Skipped invalid user-defined configs:\n"
+		var errorsComment strings.Builder
+		errorsComment.WriteString("# Skipped invalid user-defined configs:\n")
 		for _, e := range configErrors {
-			errorsComment += fmt.Sprintf("# - %s\n", e)
+			errorsComment.WriteString(fmt.Sprintf("# - %s\n", e))
 		}
 
-		caddyfile += "\n" + errorsComment
+		caddyfile += "\n" + errorsComment.String()
 	}
 
 	return caddyfileHeader + "\n" + caddyfile, nil

internal/machine/firewall/iptables_linux.go

@@ -103,7 +103,7 @@ func createIptablesChains() error {
 			}
 
 			firstRejectRuleNum := 0
-			for _, line := range strings.Split(string(out), "\n") {
+			for line := range strings.SplitSeq(string(out), "\n") {
 				fields := strings.Fields(line)
 				if len(fields) < 2 {
 					continue