Update actions/checkout action to v3

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
actions/checkout	action	major	v3	source

Test plan: CI should pass with updated dependencies. No review required: this is an automated dependency update PR.

Release Notes

Configuration

Schedule: "on the 1st through 7th day of the month" in timezone America/Los_Angeles.

Automerge: Disabled by config. Please merge this manually once you are satisfied.

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