Use checkout_release_branch from a8c-ci-toolkit

[AliSoftware]

Closes AINFRA-2828

Why?

• AINFRA-2725 standardized the .buildkite/commands/checkout-release-branch.sh script to byte-identical copies across 13 repos.
• AINFRA-2828 moves it a step further, making that script live in a8c-ci-toolkit as the checkout_release_branch command, so there is a single source of truth for its implementation.

How?

Behaviour is unchanged — the command in a8c-ci-toolkit is a straight port of the script being deleted here.
Every call site already runs in a step with the toolkit plugin loaded, which is what puts the command on $PATH.

Compatibility

The toolkit pin moves from 5.3.1 to 6.3.0. I checked the breaking changes in the toolkit's CHANGELOG.md between those two versions and none apply to this repo.

Draft Status

Important

Draft until a8c-ci-toolkit 6.3.0 is released — the pin doesn't resolve before then.

[dangermattic]

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