Update pySigma-backend-loki to latest release candidate to test compatibility

[mostafa]

This PR updates pySigma-backend-loki to v0.13.0rc1 to check compatibility with the convert action.

Notes:

• Since RC packages are prerelease, uv won't install them by default, unless we use uv lock --prerelease=allow to update the lock file and install all test latest Python deps and there's no way to select a single prerelease package to be updated.
• Splunk package is no longer compatible, because a new RC version is not yet released (as of today). Instead I manually tested pysigma-backend-elasticsearch v1.2.0rc1 and it worked as expected.

Part of #106.

[kelnage]

Question: based on that error message, do we need to do a pre-release for pySigma-backend-loki? At the moment the work to support rc is just in a branch. I've not tried an RC release on PyPI before, so I'm not sure of the work needed.

 Because only pysigma-backend-loki==0.12.4 is available and pysigma-backend-loki==0.12.4 depends
      on pysigma>=1.0.0rc1,<2.0.0, we can conclude that all versions of pysigma-backend-loki depend on
      pysigma>=1.0.0rc1,<2.0.0.

[mostafa]

@kelnage Me neither. So I guess we should figure this out together. 😄

[mostafa]

Unfortunately the splunk plugin is not yet compatible, so I had to remove it for testing.

[kelnage]

The Loki RC PR is ready for review, PTAL 🙏

[mikefat]

This PR tests how SRD would work once pySigma v1.0.0 is released. We're holding off merging it until it's official - last time checked it worked (per conversation with @kelnage)