Support pygls 2

[polyzen]

Test error:

ImportError while loading conftest '/build/systemd-language-server/src/systemd-language-server-0.3.5/tests/conftest.py'.
tests/conftest.py:7: in <module>
    from pygls.server import LanguageServer
E   ImportError: cannot import name 'LanguageServer' from 'pygls.server' (/usr/lib/python3.13/site-packages/pygls/server.py)

You get the same error if you try to run the language server with pygls 2 and lsprotocol 2025 installed.