Fix GH action warning by upgrading actions/checkout

[jsnajdr]

When inspecting GitHub action results, I accidentally noticed this warning for the "Validate Gradle Wrapper" check:

Solving it by updating the actions/checkout action declaration to v3.1.0. If you search all other references to actions/checkout in our other actions, you'll they use the same version with the same hash. The "Validate Gradle Wrapper" action was the only exception. Because it was added recently, after the last mass-upgrade of the action versions.

[tyxla]

LGTM 👍

I restarted the failing RN tests since I believe the failure shouldn't be related.

[github-actions]

Flaky tests detected in 4714826.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4024493562
📝 Reported issues:

• [Flaky Test] should be able to create an hierarchical post without title support #36939 in specs/editor/plugins/custom-post-types.test.js