NMVpnConnectionStateReason is no longer used and replaced by
NMActiveConnectionStateReason. However, the old enums should
stay in place as they were:

Otherwise:
  #define NMVpnConnectionStateReason NMActiveConnectionStateReason
causes compiler warnings:
  NMVpnConnectionStateReason x;
  x = NM_VPN_CONNECTION_STATE_REASON_UNKNOWN;             // -Wenum-conversion
  if (x == NM_VPN_CONNECTION_STATE_REASON_NO_SECRETS) { } // -Wenum-compare

Similarly, a user who didn't upgrade shall continue to get the
old GType for NM_TYPE_VPN_CONNECTION_STATE_REASON.

In practice, old users will have no issues using the old enum
the places where it worked before.
The only use of the deprecated enum is in vpn_state_changed()
signal slot of NMVpnConnection. This makes the signal slot
itself deprecated. However, NMVpnConnection is an NMObject and commonly
created within libnm itself, not by the user. It is very unlikely that
a user of libnm subclassed NMVpnConnection and makes use of the
vpn_state_changed() signal slot. So, deprecate it without replacement.

Fixes: a91369f8