Replace placeholders in licenses (#24354)

Replace #22117. Implement it in a more maintainable way.

Some licenses have placeholders e.g. the BSD licenses start with this
line:
```
Copyright (c) <year> <owner>. 
```
This PR replaces the placeholders with the correct value when initialize
a new repo.

### FAQ

- Why not use a regex?
It will be a pretty complicated regex which could be hard to maintain.

- There're still missing placeholders.
There are over 500 licenses, it's impossible for anyone to inspect all
of them alone. Please help to add them if you find any, and it is also
OK to leave them for the future.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

build/generate-go-licenses.go

@@ -35,12 +35,40 @@ func main() {
 
 	base, out := os.Args[1], os.Args[2]
 
+	// Add ext for excluded files because license_test.go will be included for some reason.
+	// And there are more files that should be excluded, check with:
+	//
+	// go run github.com/google/go-licenses@v1.6.0 save . --force --save_path=.go-licenses 2>/dev/null
+	// find .go-licenses -type f | while read FILE; do echo "${$(basename $FILE)##*.}"; done | sort -u
+	//    AUTHORS
+	//    COPYING
+	//    LICENSE
+	//    Makefile
+	//    NOTICE
+	//    gitignore
+	//    go
+	//    md
+	//    mod
+	//    sum
+	//    toml
+	//    txt
+	//    yml
+	//
+	// It could be removed once we have a better regex.
+	excludedExt := map[string]bool{
+		".gitignore": true,
+		".go":        true,
+		".mod":       true,
+		".sum":       true,
+		".toml":      true,
+		".yml":       true,
+	}
 	var paths []string
 	err := filepath.WalkDir(base, func(path string, entry fs.DirEntry, err error) error {
 		if err != nil {
 			return err
 		}
-		if entry.IsDir() || !licenseRe.MatchString(entry.Name()) {
+		if entry.IsDir() || !licenseRe.MatchString(entry.Name()) || excludedExt[filepath.Ext(entry.Name())] {
 			return nil
 		}
 		paths = append(paths, path)