feat(snacks): use snacks.dashboard as the default dashboard. moved dashboard-nvim to extras (#4832)

## Description

<!-- Describe the big picture of your changes to communicate to the
maintainers
  why we should accept this pull request. -->

Snacks has a new dashboard plugin that will be LazyVim's default.

Check the docs at
https://github.com/folke/snacks.nvim/blob/main/docs/dashboard.md

## Related Issue(s)

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  If this PR fixes any issues, please link to the issue here.
  - Fixes #<issue_number>
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## Screenshots

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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/ui/alpha.lua

@@ -1,7 +1,6 @@
 return {
 
-  { "nvimdev/dashboard-nvim", enabled = false },
-  { "echasnovski/mini.starter", enabled = false },
+  { "folke/snacks.nvim", opts = { dashboard = { enabled = false } } },
   -- Dashboard. This runs when neovim starts, and is what displays
   -- the "LAZYVIM" banner.
   {