Change default changelog filename to CHANGELOG.md

[takanuva15]

Hi, thank you for making this awesome GitHub Action. This is exactly what I was looking for in my project ever since I forgot to edit the changelog in my last few PRs.

I wanted to request whether you can change the default filename to CHANGELOG.md since that is the default filename used by a huge number of coders and organizations worldwide, and it is also the default filename processed by GitHub, GitLab, Bitbucket, and other major repository providers. In addition, the standardized specification for the Changelog format at keepachangelog.com that has been adopted by a huge number of code repositories also says to use the default filename CHANGELOG.md. (On the website, you can scroll down to see the FAQ "What should the changelog file be named?" to see their statement)