Update goharbor/notary-signer-photon Docker tag to v2.8.0

This MR contains the following updates:

Package	Type	Update	Change
goharbor/notary-signer-photon	ironbank-docker	minor	v2.7.1 -> v2.8.0
goharbor/notary-signer-photon		minor	v2.7.1 -> v2.8.0
goharbor/notary-signer-photon	stage	minor	v2.7.1 -> v2.8.0

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