Update Flit release tooling to v4

[harryswift01]

Summary

This PR updates the release tooling to Flit 4 to fix the PyPI publishing failure caused by Flit 3 resolving against the newly released flit_core 4.x. The two major versions are incompatible, resulting in flit publish failing while building the source distribution.

Changes

Flit configuration:

• Updated the release workflow to use flit>=4,<5 and flit_core>=4,<5.
• Updated the build-system requirement to flit_core>=4,<5.
• Added --use-vcs to flit publish to preserve the existing source distribution behaviour under Flit 4.

Impact

• Restores PyPI publishing by keeping flit and flit_core on compatible major versions.
• Prevents future major flit_core releases from being pulled into the current Flit release line unexpectedly.
• Preserves the existing VCS-based source distribution behaviour.