Bump ssh-agent version to 6.12.0-jdk21

[github-actions]

Bump Docker image tag for "jenkins/ssh-agent"

[jenkins/ssh-agent] Bump Docker image tag in "docker-compose.yaml"

change detected: * key "$.services.default_agent.image" updated from "jenkins/ssh-agent:6.12.0" to "jenkins/ssh-agent:6.12.0-jdk21", in file "docker-compose.yaml"

[jenkins/ssh-agent] Bump Docker image tag in "build-docker-compose.yaml"

change detected: * key "$.services.default_agent.image" updated from "jenkins/ssh-agent:6.12.0" to "jenkins/ssh-agent:6.12.0-jdk21", in file "build-docker-compose.yaml"

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