Remove des3 and arcfour from supported_enctypes

The des3 and arcfour (rc4) enctypes use weak string-to-key algorithms,
and should not be used for producing password-derived keys.

ticket: 7903
Update example enctypes in kdc_conf.rst

Use modern enctypes for values of master_key_type and
supported_enctypes in the example kdc.conf in kdc_conf.rst.

ticket: 8035 (new)
target_version: 1.13.1
tags: pullup