Bump taskcluster from 20.0.0 to 21.0.0 (#1026)

Bumps [taskcluster](https://github.com/taskcluster/taskcluster) from 20.0.0 to 21.0.0.
- [Release notes](https://github.com/taskcluster/taskcluster/releases)
- [Changelog](https://github.com/taskcluster/taskcluster/blob/master/CHANGELOG.md)
- [Commits](https://github.com/taskcluster/taskcluster/compare/v20.0.0...v21.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

infra/spawn_pipeline_requirements.txt

@@ -1,4 +1,4 @@
 json-e==3.0.0
 pyyaml==5.1.2
 requests==2.22.0
-taskcluster==20.0.0
+taskcluster==21.0.0

requirements.txt

@@ -14,7 +14,7 @@ scikit-learn==0.21.3
 scipy==1.3.1
 shap==0.30.2
 tabulate==0.8.5
-taskcluster==20.0.0
+taskcluster==21.0.0
 tqdm==4.36.1
 xgboost==0.90
 zstandard==0.12.0