Update Poetry build backend to poetry-core

https://python-poetry.org/docs/pyproject/#poetry-and-pep-517

Author: br3ndonland

{{cookiecutter.project_slug}}/backend/app/pyproject.toml

@@ -37,7 +37,7 @@ multi_line_output = 3
 include_trailing_comma = true
 force_grid_wrap = 0
 line_length = 88
-[build-system]
-requires = ["poetry>=0.12"]
-build-backend = "poetry.masonry.api"
 
+[build-system]
+requires = ["poetry-core>=1.0.0"]
+build-backend = "poetry.core.masonry.api"