build: update license identifier according to the license notice in source files

pyproject.toml

@@ -3,7 +3,7 @@ name = "yamllint"
 description = "A linter for YAML files."
 readme = {file = "README.rst", content-type = "text/x-rst"}
 requires-python = ">=3.7"
-license = {text = "GPL-3.0-only"}
+license = {text = "GPL-3.0-or-later"}
 authors = [{name = "Adrien Vergé"}]
 keywords = ["yaml", "lint", "linter", "syntax", "checker"]
 classifiers = [