drop python 3.8

[helpmefindaname]

Since python3.8 is no longer supported, we can now drop it and use python3.9 features in our code, especially improving typing.

This pr does:

• increase the minimum python version to 3.9 and uses it in gh-actions
• update ruff to version 0.7 & fixes new linting errors
• uses the python3.9 typing features
• removes some methods that aren't used since longer
• ensure that transformer embeddings will stay with the attention implementation that were saved, preventing precision issues after reloading.

[alanakbik]

@helpmefindaname thanks a lot for the PR!