refactor: cleanup GetArtifactRegistryCredentials error handling

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>

go.sum

@@ -181,8 +181,8 @@ github.com/fluxcd/pkg/apis/kustomize v1.10.0 h1:47EeSzkQvlQZdH92vHMe2lK2iR8aOSEJ
 github.com/fluxcd/pkg/apis/kustomize v1.10.0/go.mod h1:UsqMV4sqNa1Yg0pmTsdkHRJr7bafBOENIJoAN+3ezaQ=
 github.com/fluxcd/pkg/apis/meta v1.12.0 h1:XW15TKZieC2b7MN8VS85stqZJOx+/b8jATQ/xTUhVYg=
 github.com/fluxcd/pkg/apis/meta v1.12.0/go.mod h1:+son1Va60x2eiDcTwd7lcctbI6C+K3gM7R+ULmEq1SI=
-github.com/fluxcd/pkg/auth v0.17.0 h1:jgum55f5K7Db6yI2bi4WeKojTzQS9KxlHCC0CsFs5x8=
-github.com/fluxcd/pkg/auth v0.17.0/go.mod h1:4h6s8VBNuec3tWd4xIReLw8BYPOKaIegjNMEbA4ikTU=
+github.com/fluxcd/pkg/auth v0.18.0 h1:71pGdKe0PVKWQvM3hEuyd3FD9dEUHtMuKMbUeiMl4aA=
+github.com/fluxcd/pkg/auth v0.18.0/go.mod h1:4h6s8VBNuec3tWd4xIReLw8BYPOKaIegjNMEbA4ikTU=
 github.com/fluxcd/pkg/cache v0.9.0 h1:EGKfOLMG3fOwWnH/4Axl5xd425mxoQbZzlZoLfd8PDk=
 github.com/fluxcd/pkg/cache v0.9.0/go.mod h1:jMwabjWfsC5lW8hE7NM3wtGNwSJ38Javx6EKbEi7INU=
 github.com/fluxcd/pkg/chartutil v1.3.0 h1:Zoc+AIyKL4YU4PaLL/iGv9VRLujeWT2Mvj4BLGFGKlg=