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>

cmd/flux/oci.go

@@ -18,7 +18,6 @@ package main
 
 import (
 	"context"
-	"errors"
 	"fmt"
 
 	"github.com/google/go-containerregistry/pkg/crane"
@@ -38,8 +37,5 @@ func loginWithProvider(ctx context.Context, url, provider string) (crane.Option,
 	if err != nil {
 		return nil, fmt.Errorf("could not login to provider %s with url %s: %w", provider, url, err)
 	}
-	if authenticator == nil {
-		return nil, errors.New("unsupported provider")
-	}
 	return crane.WithAuth(authenticator), nil
 }