chore(deps): update docker.redpanda.com/redpandadata/redpanda docker tag to v23.3.21

[app-token-issuer-releng-renovate]

This PR contains the following updates:

Package	Update	Change
docker.redpanda.com/redpandadata/redpanda	patch	v23.3.18 -> v23.3.21

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[app-token-issuer-releng-renovate]

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