`python-language-server` deprecated, switch to `python-lsp-server`.

## Summary
[python-language-server](https://github.com/palantir/python-language-server) on which this Atom package depends is deprecated in favor of [python-lsp-server](https://github.com/python-lsp/python-lsp-server), as confirmed by this [issue](https://github.com/palantir/python-language-server/issues/935) and the long list of projects mentioning it underneath.

- The installation instructions for ide-python still mention installing the deprecated repo.
- The plugin defaults to the old version if the newer one is not found, as https://github.com/atom-community/ide-python/pull/389 shows.

## Motivation
- Allow future PLSP features to improve this package while keeping an up-to-date Langage Server backend.

## Describe alternatives you've considered
I just installed the correct version, and everything worked as expected.

Start by reviewing the installation instructions and the plugin default-selection behavior described in the issue, then compare the approach in pull request 389. Update both references from python-language-server to python-lsp-server and verify that installing the newer server works as reported.

Written by the indexing model from the issue text.