chore: upgrade self to cdktf 0.20.0 (#260)

This PR upgrades CDKTF to version `0.20.0` in `cdktf-repository-manager`
(this repo).
Unfortunately, not everything can be automated, and the following steps
need to be completed manually:

- [x] Wait for #259 to be merged and for the deployment to succeed
- [x] Upgrade `@cdktf/provider-github` to a version compatible with
`cdktf@0.20.0` (`yarn add ...`)

Please checkout this PR, complete the above steps, push the changes to
this branch, and then mark this PR as ready for review to complete the
upgrade. Thanks!

---------

Signed-off-by: team-tf-cdk <github-team-tf-cdk@hashicorp.com>
Co-authored-by: Daniel Schmidt <danielmschmidt92@gmail.com>

package.json

@@ -39,9 +39,9 @@
     ]
   },
   "dependencies": {
-    "@cdktf/provider-github": "^12.0.0",
-    "cdktf": "^0.19.0",
-    "cdktf-cli": "^0.19.0",
+    "@cdktf/provider-github": "^13.0.0",
+    "cdktf": "^0.20.0",
+    "cdktf-cli": "^0.20.0",
     "change-case": "^4.1.2",
     "constructs": "^10.0"
   },