bgp: T7760: deprecate per bgp vrf instance system-as node

Originating from the bug in T7665. To avoid potential issues down the line - and
given that there's no compelling technical reason to retain the system-as CLI
node under per-VRF BGP configuration, which cannot be achieved through
alternative means - the maintainers have collectively decided to deprecate the
following command:

set vrf name <name> protocols bgp system-as <asn>

Starting with VyOS 1.4.4, this CLI command will be considered deprecated. While
it will still be accepted, it will no longer have any operational effect. A
deprecation warning will be displayed at commit time, indicating that the BGP
ASN from the global BGP configuration is now used instead.

A migration script will handle the transition and perform the following actions:
* Ensure a global BGP configuration exists; if not, initialize one.
* Iterate over all configured VRFs to determine whether a BGP instance exists
* For any insance, update the configuration to use the global system-as
  and apply the local-as ASN no-prepend replace-as option on all affected
  neighbors to preserve existing behavior.
* If a neighbor is already configured with a local-as directive, that neighbor
  will be excluded from the migration process, as it already follows a custom
  configuration.
* Add allowas-in per neighbor option. Required to not deny prefix received
  updates due to as-path contains our own global ASN.

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