add 'application.properties' to '.gitignore'

proccess

1. edit .gitignore file

2. 'git rm --cached <관리하지 않고자하는 파일의 경로>'

3. 'git status' to check deleting correctly

4. add, commit, push

reference
https://nosy-rabbit.tistory.com/entry/git-%EC%9D%B8%ED%85%94%EB%A6%AC%EC%A0%9C%EC%9D%B4-%EC%97%90%EC%84%9C-applicationproperties%ED%8C%8C%EC%9D%BC-gitignore-%ED%95%98%EA%B8%B0
https://medium.com/@kwoncharles/git-gitignore-%EB%B3%80%EA%B2%BD-%EB%B0%98%EC%98%81%ED%95%98%EA%B8%B0-3f0559947c2b
https://dev-jwblog.tistory.com/51