upgrade getdeps GitHub actions to Ubuntu 20 (#516)

Summary:
X-link: facebook/fbthrift#516

X-link: facebook/watchman#1050

X-link: facebook/proxygen#426

X-link: facebook/folly#1842

Pull Request resolved: #117

Sadly, even though Ubuntu 18.04 is still in LTS, GitHub is deprecating
its runner image.

Migrate the generated GitHub Actions to 20.04.

actions/runner-images#6002

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Reviewed By: fanzeyi

Differential Revision: D38877286

fbshipit-source-id: 85f3324d6666eacb190a43985585b438de69d545

build/fbcode_builder/getdeps.py

@@ -1115,7 +1115,7 @@ def setup_project_cmd_parser(self, parser):
             help="Allow CI to fire on all branches - Handy for testing",
         )
         parser.add_argument(
-            "--ubuntu-version", default="18.04", help="Version of Ubuntu to use"
+            "--ubuntu-version", default="20.04", help="Version of Ubuntu to use"
         )
         parser.add_argument(
             "--main-branch",