outline_panel: Rename outline_panel::Open to outline_panel::OpenSelectedEntry (#28890)

Closes #27171

The `outline_panel::Open` action seems to open the outline panel, but
instead, it moves the editor's cursor to the position of the selected
entry in the outline panel. This PR renames it to
`outline_panel::OpenSelectedEntry` for better clarity.

Meanwhile, there is an existing action, `outline_panel::ToggleFocus`,
that should be used for opening the outline panel.

Todo:
- [x] Added migration

Release Notes:

- Renamed `outline_panel::Open` to `outline_panel::OpenSelectedEntry`
for better clarity.

assets/keymaps/default-linux.json

@@ -721,7 +721,7 @@
       "alt-shift-copy": "workspace::CopyRelativePath",
       "alt-ctrl-shift-c": "workspace::CopyRelativePath",
       "alt-ctrl-r": "outline_panel::RevealInFileManager",
-      "space": "outline_panel::Open",
+      "space": "outline_panel::OpenSelectedEntry",
       "shift-down": "menu::SelectNext",
       "shift-up": "menu::SelectPrevious",
       "alt-enter": "editor::OpenExcerpts",