Use PR title and description for commit message

[kou]

Which issue does this PR close?

Closes #7562.

Rationale for this change

We have our PR template and it has very useful information than the current default commit message:

• The commit message of the first commit if only one commit exists in the PR
• The commit title of the first commit as a title and list of commit titles of all commits if multiple commits exist in the PR

What changes are included in this PR?

Changed the default commit message to the PR title and description.

Are there any user-facing changes?

No.

[alamb]

Thank you @kou (and @mbrobbel)

I think we should give it a try!