Remove deprecated function tbl_flatten

[clason]

In Nvim 0.10, vim.tbl_flatten was deprecated in favor of vim.iter(...):flatten().totable() (see :checkhealth vim.deprecated warning after using the plugin). You can either add a backward compatibility shim that checks for vim.iter, or just raise the minimum version to Nvim 0.10.

(On a related note, I'd suggest explicitly stating the minimum supported Neovim version in the Readme.)

[delphinus]

The upstream telscope's issue seems not to be processed. I start to think remove these deprecation warnings by bumping up this plugin's minimal requirements to v0.10.0.