Migrate to pyproject.toml and hatchling

[pquentin]

As done in elastic/elasticsearch-serverless-python#6, except that:

• There was no need to fix Dockerfiles or docs configuration
• I use elasticsearch/_version.py as the source of truth for the version, which avoids keeping the version in two places
• I fixed utils/build-dists.py to use pyproject.toml.

[github-actions]

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