chore: delete release branch if exists

If it exists, the git command failed, and the current branch is
not changed.

Author: kenjis

admin/prepare-release.php

@@ -26,6 +26,7 @@ function replace_file_content(string $path, string $pattern, string $replace): v
 
 // Creates a branch for release.
 system('git switch develop');
+system('git branch -D release-' . $version);
 system('git switch -c release-' . $version);
 
 // Updates version number in "CodeIgniter.php".