Update mitre/heimdall2 Docker tag to v2.6.1

This MR contains the following updates:

Package	Type	Update	Change
mitre/heimdall2	ironbank-docker	minor	2.5.3 -> 2.6.1
mitre/heimdall2		minor	2.5.3 -> 2.6.1
mitre/heimdall2	stage	minor	2.5.3 -> 2.6.1

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