[release-16.0] Upgrade the Golang version to go1.20.3

[github-actions]

This Pull Request bumps the Golang version to go1.20.3 and the bootstrap version to 14.3.

Do not trust the bot blindly. A thorough code review must be done to ensure all the files have been correctly modified.

There are a few manual steps remaining:

• Make sure you update the Golang version used in the previous and next release branches for the Upgrade/Downgrade tests.
  • Use go1.20.3 in the upgrade downgrade tests #12839
• Build and Push the bootstrap images to Docker Hub, the bot cannot handle that.
• Update the ./.github/workflows/*.yml files with the newer Golang version, the bot cannot handle that due to permissions.
  • To accomplish this, run the following: go run ./go/tools/go-upgrade/go-upgrade.go upgrade workflows --go-to=1.20.3

[vitess-bot]

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