Use /healthz HTTP probes and graceful termination settings

[gaul]

S3Proxy has served an unauthenticated GET /healthz since 3.2.0, within this chart's appVersion (3.3.0), so the probes can do better than a TCP accept check:

• Probes: switch tcpSocket to httpGet /healthz, which distinguishes a serving proxy from one that merely holds a listen socket open (a wedged JVM passes a tcpSocket probe indefinitely). When config.tls.enabled, the probes use scheme: HTTPS; the kubelet does not verify certificates for HTTPS probes, so self-signed keystores keep working — addressing the concern in the original tcpSocket comment.
• preStop sleep 5: endpoint removal propagates asynchronously on pod deletion; serving briefly after the deletion mark keeps late-routed connections from landing on a pod that has already begun shutting down.
• terminationGracePeriodSeconds: 40 (new documented value): covers the preStop sleep plus the up-to-30-second in-flight request drain that S3Proxy performs on SIGTERM starting with releases after 3.3.0 (gaul/s3proxy@d637104f). On those releases a terminating pod also fails readiness while draining, which the httpGet readiness probe surfaces — pods leave Service endpoints while transfers finish instead of dropping them.

Housekeeping per CONTRIBUTING: chart version bumped to 0.4.0, the new value carries a helm-docs # -- comment, and README.md is left untouched for the release automation.

Validated locally: helm lint clean; helm template across the test-values/ scenarios (including both TLS scenarios, which are the only renders emitting scheme: HTTPS); kubeconform -strict passes on the rendered default and TLS manifests (12/12 resources).

Context: s3proxy recently gained container-focused behavior this chart can lean on — the shutdown drain above and reference manifests at examples/kubernetes, which link this chart as the Helm option. The new *_FILE credential variables were considered and are deliberately not used here: the merge-configs initContainer already keeps credentials in mounted files rather than env vars or argv.

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