chore: use kafka-backup v0.15.6 by default

[sionsmith]

Summary

• bump the Strimzi operator release metadata to 0.2.5
• update the default backup/restore job image from osodevops/kafka-backup:v0.15.5 to osodevops/kafka-backup:v0.15.6
• regenerate the CRDs so the spec.image descriptions show the new default

Why

Unlike kafka-backup-operator, this operator does not link kafka-backup-core directly. It creates Jobs/CronJobs that run the kafka-backup CLI image, so the core v0.15.6 fix is only picked up by default after this image bump.

Users can still override spec.image, but the default should point at the fixed CLI release.

Local verification

• cargo check
• cargo fmt --all -- --check
• cargo test --all
• cargo clippy --all-targets --all-features -- -D warnings
• helm lint deploy/helm/strimzi-backup-operator
• helm template test-release deploy/helm/strimzi-backup-operator >/tmp/strimzi-backup-operator-helm.yaml
• scripts/release-gate.sh
• cargo run --bin crdgen
• verified generated CRDs match Helm CRDs