Drop support for Python 3.9

Came up during 0.43.0 RC1 release vote:

In a clean Python 3.9 container, installing the RC1 wheel with the [cli] extra
succeeds, but both `burr --help` and `import burr.cli.__main__` fail with:

TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

The failure comes from `dict | None` in burr/cli/__main__.py, which requires
Python 3.10+. The release-validation workflow explicitly skips Python 3.9 because of this known incompatibility.

Python 3.9 is EOL so I suggest we just drop the official support for this.

Start with burr/cli/__main__.py and reproduce the failure in a clean Python 3.9 container using the [cli] extra, then inspect the release-validation workflow that skips Python 3.9. Done means official support is removed consistently and the supported-version validation no longer treats Python 3.9 as supported.

Written by the indexing model from the issue text.