T8515: remove pre-1.3 deprecated code artifacts from vyos_bgp_global (#461)

• T8515: remove commented-out pre-1.3 parameter artifacts from vyos_bgp_global docs

• T8515: remove stale vrf.old backup file

• T8515: add changelog fragment for deprecated code cleanup

• T8515: fix grammar in ebgp_multihop description

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: omnom62 <75066712+omnom62@users.noreply.github.com>