Migrate from deprecated `pdm-pep517` to `pdm-backend` for build

pyproject.toml

@@ -5,7 +5,7 @@ description = ""
 authors = [
     {name = "Adam Goldsmith", email = "adam@adamgoldsmith.name"},
 ]
-license = "MIT"
+license = {text = "MIT"}
 dependencies = [
     "discord-py~=2.2",
     "aiohttp~=3.8",
@@ -25,8 +25,8 @@ format = [
 ]
 
 [build-system]
-requires = ["pdm-pep517>=0.12.0"]
-build-backend = "pdm.pep517.api"
+requires = ["pdm-backend"]
+build-backend = "pdm.backend"
 
 [tool.black]