Update openpolicyagent/opa Docker tag to v0.31.0

This MR contains the following updates:

Package	Type	Update	Change
openpolicyagent/opa	ironbank-docker	minor	0.30.2 -> 0.31.0
openpolicyagent/opa		minor	0.30.2 -> 0.31.0
openpolicyagent/opa	stage	minor	0.30.2 -> 0.31.0

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