kubeapiserver: rename '--experimental-encryption-provider-config' to '--encryption-provider-config'.

This change renames the '--experimental-encryption-provider-config'
flag to '--encryption-provider-config'. The old flag is accepted but
generates a warning.

In 1.14, we will drop support for '--experimental-encryption-provider-config'
entirely.
Co-authored-by: Stanislav Laznicka <slaznick@redhat.com>