OKTA-1115736 Update passkey authenticator label to singular

[mpeng-okta]

Description:

Update the Passkey authenticator label from plural "Passkeys" to singular "Passkey" in login.properties for consistency with other Okta authenticators (e.g., "Okta Verify", "Google Authenticator", "YubiKey").

• oie.webauthn.passkeysRebrand.passkeys.label: "Passkeys" → "Passkey"

Companion PRs: admin-mfa#164, okta-core#126916

PR Checklist

• Have you verified the basic functionality for this change?
• Did you add tests, as appropriate, following our Automated Test guidelines?
• Did you follow our Security Best Practices?
• Did you verify the change by running downstream monolith artifact? (Provide link to build below)
• Does this PR include noticeable changes to the UI? (If yes, attach screenshots/video below)

Issue:

• OKTA-1115736

Reviewers:

Screenshot/Video:

Downstream Monolith Build:

[alexchen-okta]

you may need to generate the ok_PL and ok_SK changes too. I believe you can do this via yarn pseudo-loc

[alexchen-okta]

Is this a dupe of #4006?