Update apache/nifi Docker tag to v1.15.0

This MR contains the following updates:

Package	Type	Update	Change
apache/nifi	ironbank-docker	minor	1.14.0 -> 1.15.0
apache/nifi		minor	1.14.0 -> 1.15.0
apache/nifi	stage	minor	1.14.0 -> 1.15.0

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