Update registry1.dso.mil/ironbank/opensource/postgres/postgresql12 Docker tag to v12.15

This MR contains the following updates:

Package	Type	Update	Change
registry1.dso.mil/ironbank/opensource/postgres/postgresql12	ironbank-base	minor	12.14 -> 12.15
registry1.dso.mil/ironbank/opensource/postgres/postgresql12	final	minor	12.14 -> 12.15

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