fix(storage): drop imagePullSecrets from OCI auth

[pujitha24]

Changes

When a TaskRun/PipelineRun's ServiceAccount has both imagePullSecrets
and mounted secrets targeting the same OCI registry, the vendored
go-containerregistry keychain consolidates both sets and returns the
first match, which is always the read-only imagePullSecrets
credential. The OCI storage backend (pkg/chains/storage/oci/legacy.go,
still the active auth path wired in pkg/chains/storage/storage.go
despite its "Deprecated" doc-comment on the Backend type) only ever
pushes signed attestations, so resolving a read-only credential makes
the push fail with an auth error even though a push-capable credential
is available in secrets.

This addresses the ServiceAccount-level case described in the issue;
the PodTemplate-level case was already fixed in a prior, separate
change.

Approach

• Bump the vendored github.com/google/go-containerregistry
pkg/authn/kubernetes module to the commit that merged upstream's
  IgnorePullSecrets option
  (feat(kubernetes): allow ignoring pull secrets google/go-containerregistry#2315), which
  lets the keychain skip both the ServiceAccount's imagePullSecrets
  and any explicit pull secrets.
• Set IgnorePullSecrets: true when building the k8schain options used
  by the OCI backend, since this keychain is only ever used to push,
  never to pull.

Validation

• go build ./... passes.
• make test-unit passes for all packages.
• make golangci-lint PKG=./pkg/chains/storage/oci/... reports 0 issues.
• Added TestK8schainOptions_IgnoresImagePullSecrets, confirmed to fail
  if IgnorePullSecrets: true is removed and to pass with the fix,
  asserting the OCI backend's k8schain options are built with
  IgnorePullSecrets set.
• Not run: no live cluster is available in this environment, so the fix
  was not reproduced end-to-end against a real registry. Correctness
  instead relies on the upstream library's own test coverage for
  IgnorePullSecrets plus the added unit test confirming this backend
  threads the option through.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

When 'oci' is configured as the storage option for signed artifacts,
the tekton-chains-controller no longer uses the ServiceAccount's
`imagePullSecrets` to authenticate with the target OCI registry when
that ServiceAccount also has push-capable `secrets` for the same
registry.

---

AI assistance: this change was drafted with Claude Code.

Fixes #1336

[tekton-robot]

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