Update vendored dependencies for oadp-1.6 alignment

[Joeavaikath]

Summary

• Update external-snapshotter client from v7 to v8 (volumegroupsnapshot/v1beta1)
• Update vendored velero packages (APIs, plugin framework, internal credentials/resourcepolicies, utility packages)
• Update vendored grpc to latest version

Why

Aligns the vendor directory with go.mod dependency changes for the oadp-1.6 branch. The previous vendor state was inconsistent with the module requirements after the go.mod update in #229.

Test plan

• Verify go build ./... succeeds with updated vendor
• Verify go vet ./... passes
• Run existing unit tests to confirm no regressions
• Confirm CI passes

🤖 Generated with Claude Code

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[Joeavaikath]

#231