trying not to violate the principle of least astonishment.
we want to only auto-enable PKCE on *new* auth methods,
rather than *new or updated* auth methods, to avoid a
scenario where a Nomad admin updates an auth method
sometime in the future -- something innocent like a new
client secret -- and their OIDC provider doesn't like PKCE.
the main concern is that the provider won't like PKCE
in a totally confusing way. error messages rarely
say PKCE directly, so why the user's auth method
suddenly broke would be a big mystery.
this means that to enable it on existing auth methods,
you would set `OIDCDisablePKCE = false`, and the double-
negative doesn't feel right, so instead, swap the language,
so enabling it on *existing* methods reads sensibly, and to
disable it on *new* methods reads ok-enough:
`OIDCEnablePKCE = false`
