Update fluxcd/pkg/auth to v0.18.0 and simplify error handling for
GetArtifactRegistryCredentials() following the improvements made in
the library.
Similar to fluxcd/image-reflector-controller#786, this removes
unnecessary nil checks as the function now returns errors directly
for unsupported providers.
- Replace authentication code in push_artifact.go with loginWithProvider()
- Remove unnecessary authenticator nil check in oci.go
- Remove unused imports (errors, auth packages)
Signed-off-by: cappyzawa <cappyzawa@gmail.com>
