chore(camel-test-infra-keycloak): upgrade keycloak.container to nightly

[github-actions]

This PR updates the keycloak.container container image to version nightly.

Update Details

• Property: keycloak.container
• Image: mirror.gcr.io/keycloak/keycloak
• File: /home/runner/work/camel/camel/test-infra/camel-test-infra-keycloak/src/main/resources/org/apache/camel/test/infra/keycloak/services/container.properties
• Old version: 26.3.5
• New version: nightly

Verification

Please verify:

• Container image version is compatible with existing tests
• No breaking changes in the updated version
• Tests pass with the new version

Run the following to verify:

mvn clean verify -pl camel-test-infra-keycloak

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This PR was automatically created by the Container Version Upgrade workflow.