Deprecated "-alt" keybindings are still defined internally and may conflict with actual user-defined keybinds.

### Describe the bug
Because the (now deprecated) "-alt" keybindings (e.g. `prevItem-alt` are still defined internally _and have a default key set_, it's possible that a keybinding the user explicitly sets is instead invisibly overwritten.

### To Reproduce
Steps to reproduce the behavior:

1. Set the following minimal "config.yml":
```yaml
keybinding:
universal:
prevItem: i
nextItem: k
```
2. `lazygit`
3. Activate the "commits" pane.
4. Press 'k'.
5. "Nothing" happens.

What's really happening here is you're triggering the hidden "prevItem-alt" keybinding, which defaults to 'k' and overrides your "nextItem" setting. (You can see this if you click on a commit down the list and _then_ press 'k', which moves your cursor **up** a line instead of down.)

Per [the current docs](https://github.com/jesseduffield/lazygit/blob/master/docs/Config.md) (on both the "master" branch and the "v0.63.0" tag), the above config is how you set the "prevItem" and "nextItem" keybinds, but the (now undocumented but still active) "-alt" keybinds are working against you.

My suggestion would be to either:
- fully remove (rather than just deprecating) the old "-alt" keybinds, or ...
- deprioritize (I'm guessing this comes down to what order the config is processed in?) the "-alt" keybinds so that user-set bindings always override them

### Version info:

* `commit=ce5a8b61bdc9439f2137a1ce9c6a5981c412a0ad, build date=2026-07-04T12:45:51Z, build source=binaryRelease, version=0.63.0, os=linux, arch=amd64, git version=2.53.0`

### Terminal info:
I'm seeing this on a TTY login. No GUI = no terminal emulator.

### Additional context
I've verified this is still an issue even if you define the "prevItem" and "nextItem" as arrays, FWTW:
```yaml
keybinding:
universal:
prevItem: [i]
nextItem: [k]
```

Start by reproducing the conflict with the minimal config.yml from the issue, then review docs/Config.md and the keybinding configuration handling. Done means an explicitly configured nextItem binding such as k is honored instead of being intercepted by the deprecated prevItem-alt binding; add or update coverage for that behavior if the relevant test location is found.

Written by the indexing model from the issue text.