Update percona/percona-backup-mongodb Docker tag to v2.8.0

This MR contains the following updates:

Package	Type	Update	Change
percona/percona-backup-mongodb		minor	2.7.0 -> 2.8.0
percona/percona-backup-mongodb	ironbank-docker	minor	2.7.0 -> 2.8.0
percona/percona-backup-mongodb	stage	minor	2.7.0 -> 2.8.0

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