gitsigns: use buf keymap option on Neovim 0.12.1+, fallback to buffer on older versions

[dam9000]

The recent move from keymap.set buffer to buf broke neovim on ubuntu 25.10 with:

vim/keymap.lua:0: invalid key: buf

nvim on ubuntu 25.10 is: v0.12.0-dev
So recent enough but not quite 0.12.0 release - it's an earlier pre-release snapshot, so it breaks.
I think we could make an effort to keep the support of such a version, hence this PR.
There are 2 commits:

    gitsigns re-introduce back the map helper so that buf/buffer can be runtime selected

and:

    gitsigns: use `buf` keymap option on Neovim 0.12.1+
    
    Follows the upstream rename of `buffer` to `buf` (neovim/neovim#38360,
    first shipped in v0.12.0); `buffer` is soft-deprecated, removal
    planned in v0.15. Fall back to `buffer` on older Neovim.
    
    The 0.12.1 floor (not 0.12.0) avoids distro snapshot builds like
    Ubuntu 25.10 `0.12.0~ubuntu1` (git snapshot d62bbe24cb), which reports
    `0.12.0-dev` but predates the `buf` support and rejects it with
    `invalid key: buf`.

[oriori1703]

Sorry, but considering that we say that we only the support the latest stable version, and the fact that ubuntu 25.10 is already EOL since July 9th (because interim release are only supported for 9 month), I think it's best to just use buf and avoid this complexity.

I'm closing the PR for now, but if you strongly disagree I can reevaluate.