docs(readme): update plugin manager example snippets

- Updated tag to latest release in install snippets.
- Removed `branch = 0.1.x` as this branch no longer seems to be maintained.
- Premoved packer.nvim example, as it has been archived.

@clason it appears you have taken over maintainership of this project.
What's your current recommendation? Do you plan on releasing more frequently going forward? Or should users fetch the latest `HEAD`, given that previous releases have become quite outdated?

Depending on your release strategy, it might make sense to either remove the version from the install snippets, or to add a workflow that auto-updates them.

Author: mrcjkb

README.md

@@ -84,42 +84,28 @@ Using [vim-plug](https://github.com/junegunn/vim-plug)
 
 ```viml
 Plug 'nvim-lua/plenary.nvim'
-Plug 'nvim-telescope/telescope.nvim', { 'tag': '0.1.8' }
-" or                                , { 'branch': '0.1.x' }
+Plug 'nvim-telescope/telescope.nvim', { 'tag': '0.1.9' }
 ```
 
 Using [dein](https://github.com/Shougo/dein.vim)
 
 ```viml
 call dein#add('nvim-lua/plenary.nvim')
-call dein#add('nvim-telescope/telescope.nvim', { 'rev': '0.1.8' })
-" or                                         , { 'rev': '0.1.x' })
-```
-
-Using [packer.nvim](https://github.com/wbthomason/packer.nvim)
-
-```lua
-use {
-  'nvim-telescope/telescope.nvim', tag = '0.1.8',
--- or                            , branch = '0.1.x',
-  requires = { {'nvim-lua/plenary.nvim'} }
-}
+call dein#add('nvim-telescope/telescope.nvim', { 'rev': '0.1.9' })
 ```
 
 Using [lazy.nvim](https://github.com/folke/lazy.nvim)
 
 ```lua
 -- init.lua:
     {
-    'nvim-telescope/telescope.nvim', tag = '0.1.8',
--- or                              , branch = '0.1.x',
-      dependencies = { 'nvim-lua/plenary.nvim' }
+    'nvim-telescope/telescope.nvim', tag = '0.1.9',
+     dependencies = { 'nvim-lua/plenary.nvim' }
     }
 
 -- plugins/telescope.lua:
 return {
-    'nvim-telescope/telescope.nvim', tag = '0.1.8',
--- or                              , branch = '0.1.x',
+    'nvim-telescope/telescope.nvim', tag = '0.1.9',
       dependencies = { 'nvim-lua/plenary.nvim' }
     }
 ```