Drop support for Python 3.9

[gchqdev227]

This MR officially drops support for 3.9, making the following changes:

• Remove all references to 3.9 in the metadata, and set the requires-python flag to ">=3.10".
• Remove 3.9 from the CI.
• Ran pyupgrade across the codebase.
• Removed two blocks of code with specific logic for Python 3.9.
• Also updated the Python 3.14 version in CI to the latest release candidate.