Add .git-hooks to the git ignore list

When following https://github.com/spring-projects/spring-boot/wiki/Working-with-Git-branches,
a new directory named .git-hooks is created when using git worktree support.
This directory should be on the ignore list.

Author: mhalbritter

.gitignore

@@ -39,3 +39,4 @@ target
 secrets.yml
 .gradletasknamecache
 .sts4-cache
+.git-hooks/