Update mitre/heimdall2 Docker tag to v2.6.20

This MR contains the following updates:

Package	Type	Update	Change
mitre/heimdall2	ironbank-docker	patch	2.6.18 -> 2.6.20
mitre/heimdall2		patch	2.6.18 -> 2.6.20
mitre/heimdall2	stage	patch	2.6.18 -> 2.6.20

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