Update mappings to be compatible with AstroNvim v3

[mifix]

Since v3.0 of AstroNvim the plugin 'bufferline.nvim' isn't used anymore.

• New tab: <leader>bb is already used in v3 ("Select buffer from tabline"). Change to <leader>bn
• Pick to close: <leader>bc is already used in v3 ("Close all buffers except current"). Change to <leader>bD
• Pick to jump: already coverd in v3 with <leader>bb
• Sort by tabs: Not necessary anymore (?) because v3 scopes buffers according to tabs