Deprecate requirements.txt files

[mogres]

Come to think of it, what's the use case for these files now that we use uv and pyproject.toml? Are we keeping requirements.txt around so the codebase remains pip-compatible for users who don't have uv installed? If that's helpful for users, then it makes sense to keep them, otherwise we could consider deprecating them gradually.

Good point @rugeli! The requirements.txt files are not strictly necessary as pip install works with pyproject.toml. However, I think some of our docker workflow and CI uses the pip install workflow reading from the requirements files. How about this:

0. Merge this PR for now to see if that addresses any of the reported dependabot dependency issues.
1. We look for and update instances of pip install from requirementes files in the codebase and in the CI to use uv sync or pip install from pyproject.toml instead
2. We stop tracking the requirements.txt files, and get rid of the automated action that exports them after dependencies change.
3. Update README.md to reflect the new installation instructions

Originally posted by @mogres in #458 (comment)