bgpd:support tcp-mss for neighbor group (backport #17341)

[mergify]

support tcp-mss for neighbor group

---

This is an automatic backport of pull request #17341 done by Mergify.

[mergify]

Cherry-pick of 9fa56a0 has failed:

On branch mergify/bp/stable/10.0/pr-17341
Your branch is up to date with 'origin/stable/10.0'.

You are currently cherry-picking commit 9fa56a03c7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   bgpd/bgp_vty.c
	modified:   tests/topotests/bgp_tcp_mss/r1/bgpd.conf
	modified:   tests/topotests/bgp_tcp_mss/r1/zebra.conf
	modified:   tests/topotests/bgp_tcp_mss/r2/bgpd.conf
	modified:   tests/topotests/bgp_tcp_mss/r2/zebra.conf

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   bgpd/bgpd.c
	both modified:   tests/topotests/bgp_tcp_mss/test_bgp_tcp_mss.py

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally