Remove obsolete COSIGN_EXPERIMENTAL=1 from keyless cosign attest example (keyless is default since cosign v2)

[mailnike]

Page

content/en/policy-controller/sample-policies.md (around line 101)

Problem

The page instructs readers to export COSIGN_EXPERIMENTAL=1 immediately before a keyless cosign attest example. This environment variable is no longer required for keyless signing/attestation as of cosign v2, so the step is misleading for anyone on a current cosign release.

Evidence (from sigstore/cosign CHANGELOG.md)

• COSIGN_EXPERIMENTAL=1 is no longer required to have identity-based ("keyless") signing and transparency.
• Removing the COSIGN_EXPERIMENTAL environment variable, so the default signing method is now keyless signing with Fulcio.

In current cosign the variable only survives for an unrelated OCI 1.1 experimental registry mode, not for keyless signing/attest.

Suggested fix

Remove the export COSIGN_EXPERIMENTAL=1 line from the keyless cosign attest example (and any nearby keyless examples that still show it). I am happy to open a PR if this is agreed.