Remove yaml fork

[jesseduffield]

We have a yaml fork which specially handles null values when merging yaml files. This allows us to set a keybinding to null and have it unmapped, rather than being set to the default.

I want to migrate users' configs to instead use <disabled> in place of null so that we get the same merging behaviour without needing to use my fork.

We can test the behaviour with this integration test:

// pkg/integration/tests/misc/null_keybinding.go
package misc

import (
	"github.com/jesseduffield/lazygit/pkg/config"
	. "github.com/jesseduffield/lazygit/pkg/integration/components"
)

var NullKeybinding = NewIntegrationTest(NewIntegrationTestArgs{
	Description:  "Confirms You can set keybindings to blank to disable them",
	ExtraCmdArgs: []string{},
	Skip:         false,
	SetupConfig: func(config *config.AppConfig) {
		config.UserConfig.Keybinding.Universal.ScrollRight = "<disabled>"
		config.UserConfig.Keybinding.Universal.NextTab = "H"
	},
	SetupRepo: func(shell *Shell) {},
	Run: func(t *TestDriver, keys config.KeybindingConfig) {
		t.Views().Files().
			IsFocused().
			Press("H")

		// NOTE: by the time we add this test, we might have inserted a 'worktrees' tab between the files and submodules tab, in which case we'll need to verify here that the worktrees view is focused
		t.Views().Submodules().IsFocused()
	},
})

[stefanhaller]

Fixed by #3459.