chore(deps): update actions/checkout action to v3 (#349)

Source-Link: https://github.com/googleapis/synthtool/commit/ca879097772aeec2cbb971c3cea8ecc81522b68a
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:6162c384d685c5fe22521d3f37f6fc732bf99a085f6d47b677dbcae97fc21392
3 files changed