Replace lgtm badge showing "invalid" status with CodeQL badge

The lgtm service was switched to CodeQL GH workflow in #653

README.md

@@ -7,7 +7,7 @@
 [![Code Climate](https://codeclimate.com/github/NatLibFi/Annif/badges/gpa.svg)](https://codeclimate.com/github/NatLibFi/Annif)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/NatLibFi/Annif/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/NatLibFi/Annif/?branch=master)
 [![codebeat badge](https://codebeat.co/badges/e496f151-93db-4f0e-9e30-bc3339e58ca4)](https://codebeat.co/projects/github-com-natlibfi-annif-master)
-[![LGTM: Python](https://img.shields.io/lgtm/grade/python/g/NatLibFi/Annif.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/NatLibFi/Annif/context:python)
+[![CodeQL](https://github.com/NatLibFi/Annif/actions/workflows/codeql.yml/badge.svg)](https://github.com/NatLibFi/Annif/actions/workflows/codeql.yml)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=NatLibFi_Annif&metric=alert_status)](https://sonarcloud.io/dashboard?id=NatLibFi_Annif)
 [![docs](https://readthedocs.org/projects/annif/badge/?version=latest)](https://annif.readthedocs.io/en/latest/index.html)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)