Use uv `centralized-project-envs` preview instead of `UV_PROJECT_ENVIRONMENT`

When https://github.com/astral-sh/uv/pull/18214 is released, then we will be able to get uv to store its venvs in its cache directory. This means we should be able to remove the following settings:
https://github.com/DiamondLightSource/python-copier-template/blob/7fa903a148c16baa5e14371ded403f61cc45f8fd/.devcontainer/devcontainer.json#L16-L29

This ticket is to try this out when a new uv is released and available in `ubuntu-devcontainer`

Start with .devcontainer/devcontainer.json, especially the settings linked at lines 16-29, and check the uv centralized-project-envs preview once the required uv release is available in ubuntu-devcontainer. Remove the obsolete settings and verify that uv stores project virtual environments in its cache directory.

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