Update jhipster/jhipster-registry Docker tag to v6.8.0

This MR contains the following updates:

Package	Type	Update	Change
jhipster/jhipster-registry	ironbank-docker	minor	v6.7.1 -> v6.8.0
jhipster/jhipster-registry		minor	v6.7.1 -> v6.8.0
jhipster/jhipster-registry	stage	minor	v6.7.1 -> v6.8.0

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