Update pyproject.toml license per #1081 (#1083)

Author: BenRKarl

pyproject.toml

@@ -22,7 +22,8 @@ version = "31.2.0"
 description = "Client library for the Google Ads API"
 readme = "./README.rst"
 requires-python = ">=3.9, <3.15"
-license = { file = "LICENSE" }
+license = "Apache-2.0"
+license-files = ["LICENSE"]
 authors = [
     {name = "Google LLC", email = "googleapis-packages@google.com"}
 ]