Update Rust crate once_cell to 1.21.3 (#8127)

Author: oxide-renovate[bot]

Cargo.toml

@@ -564,7 +564,7 @@ oxide-client = { path = "clients/oxide-client" }
 oxide-vpc = { git = "https://github.com/oxidecomputer/opte", rev = "9e79588b11a69ff3d40f9aa29af97e31a0045c4b", features = [ "api", "std" ] }
 oxlog = { path = "dev-tools/oxlog" }
 oxnet = "0.1.1"
-once_cell = "1.20.3"
+once_cell = "1.21.3"
 openapi-lint = { git = "https://github.com/oxidecomputer/openapi-lint", branch = "main" }
 openapi-manager-types = { path = "dev-tools/openapi-manager/types" }
 openapiv3 = "2.0.0"