Set default signature algorithm to `SHA-256`

[const-cloudinary]

Brief Summary of Changes

What does this PR address?

• [ ] GitHub issue (Add reference - #XX)
• [ ] Refactoring
• [ ] New feature
• [x] Bug fix
• [ ] Adds more tests

Are tests included?

• [x] Yes
• [ ] No

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all of the tests pass.