ci: use CodeQL instead of LGTM
ci: use CodeQL instead of LGTM

As LGTM is going to be shut down by EOY[0], let's move the code scanning to
CodeQL as recommended. Thanks to GH integration the results from such
scans will be shown both in the respective PR and in the Security ->
Code Scanning tab[1].

[0] https://github.blog/2022-08-15-the-next-step-for-lgtm-com-github-code-scanning/
[1] https://github.com/systemd/mkosi/security/code-scanning

systemd/mkosi,systemd/mkosi