Keycloak documentation update for UI credentials

General MR

Summary

Update Keycloak documentation Issue #1985.

The documented configurations for setting custom username and password for the keycloak UI are incorrect. The current documentation has the value pathing incorrectly set as secrets.credentials.stringData.* the correct value path is secrets.env.stringData.*. The changes were validated in a dev cluster. All pertaining examples and docs have been updated.

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