modernize: update GitHub Actions and modernize Go code with latest stdlib features

## CHANGES

- Upgrade GitHub Actions to latest versions (v6, v21)
- Add modernization check step in CI workflow
- Replace strings manipulation with `strings.CutPrefix` and `strings.CutSuffix`
- Replace manual loops with `slices.Contains` for validation
- Use `strings.SplitSeq` for iterator-based string splitting
- Replace `bytes.TrimPrefix` with `bytes.CutPrefix` for clarity
- Use `strings.Builder` instead of string concatenation
- Replace `fmt.Sprintf` with `fmt.Appendf` for efficiency
- Simplify padding calculation with `max` builtin

cmd/generate_changelog/internal/changelog/generator.go

@@ -574,8 +574,8 @@ func (g *Generator) extractChanges(pr *github.PR) []string {
 	}
 
 	if len(changes) == 0 && pr.Body != "" {
-		lines := strings.Split(pr.Body, "\n")
-		for _, line := range lines {
+		lines := strings.SplitSeq(pr.Body, "\n")
+		for line := range lines {
 			line = strings.TrimSpace(line)
 			if strings.HasPrefix(line, "- ") || strings.HasPrefix(line, "* ") {
 				change := strings.TrimPrefix(strings.TrimPrefix(line, "- "), "* ")

cmd/generate_changelog/internal/changelog/processing.go

@@ -159,7 +159,7 @@ func (g *Generator) CreateNewChangelogEntry(version string) error {
 	for _, file := range files {
 		// Extract PR number from filename (e.g., "1640.txt" -> 1640)
 		filename := filepath.Base(file)
-		if prNumStr := strings.TrimSuffix(filename, ".txt"); prNumStr != filename {
+		if prNumStr, ok := strings.CutSuffix(filename, ".txt"); ok {
 			if prNum, err := strconv.Atoi(prNumStr); err == nil {
 				processedPRs[prNum] = true
 				prNumbers = append(prNumbers, prNum)