drop support for python 3.8

[baywet]

Python 3.8 has fallen out of support 2 days ago, which means it's not getting anymore security updates.
This also has consequences for dependency management since an increasing number of dependencies have moved away from it, and we're not able to get the new versions anymore.
Consider making this change at the same time as #341, and after #302 has been implemented.

TODO:

• remove 3.8 from the toml file
• remove 3.8 from the matrix of the workflows
• remove 3.8 from the policy definition if add github branch policy definition #327 has been implemented.

IMPORTANT: this is a source breaking change for 3.8 users (they'll need to upgrade to a newer version of python), make sure to include a bang in your message commit (e.g. feat!:) so a major version bump is made.

We do not need to major bump kiota itself since the generator will emit for the same API surface and we're within our support policy.

After the PR for this is merged but BEFORE the changes are released we also need to go back to any closed (not merged) dependabot PR and ask the bot to recreate the pull requests (@dependabot recreate) so we upgrade to the latest version of the dependencies.

[baywet]

additionally this documentation needs to be updated https://learn.microsoft.com/en-us/openapi/kiota/quickstarts/python#required-tools

[baywet]

And this one https://learn.microsoft.com/en-us/openapi/kiota/tutorials/python-azure?tabs=portal