Migrate package metadata from setup.py to pyproject.toml (PEP 621)

## Background

The package metadata still lives in `setup.py` (name, version, dependencies, extras, entry points, classifiers, etc.). `pyproject.toml` currently only carries tool configuration (coverage, pytest, mypy, ruff).

With the tox/uv migration landing (see #6325, #6327, #6367, and the rest of the `chore/migrate-to-tox-uv` work), the distribution itself is the last piece still using legacy setuptools configuration. Moving to PEP 621 `[project]` metadata would:

- Make the project introspectable by modern tooling (`uv sync`, `uv add`, `pip install .` without executing arbitrary Python, build frontends, dependency scanners, SBOM tools).
- Eliminate the `setup.py` execution step during builds.
- Align with the direction setuptools itself recommends.

## Scope

Port everything currently in `setup.py` into `pyproject.toml`:

- [ ] `[build-system]` table (setuptools backend)
- [ ] `[project]` table: `name`, `version`, `description`, `readme`, `requires-python`, `license`, `authors`, `urls`, `classifiers`
- [ ] `[project] dependencies` ← `install_requires`
- [ ] `[project.optional-dependencies]` ← `extras_require` (all ~40 extras — names must match exactly; they're public API)
- [ ] `[project.entry-points.\"opentelemetry_propagator\"]` ← `entry_points`
- [ ] `[tool.setuptools] zip-safe = false`
- [ ] `[tool.setuptools.package-data]` for `py.typed` (PEP 561)
- [ ] `[tool.setuptools.packages.find]` with `tests*` excluded
- [ ] Verify `MANIFEST.in` still behaves correctly (or fold into `include-package-data`)
- [ ] Update release tooling (`.craft.yml`, version-bump scripts) to read/write the version in `pyproject.toml` instead of `setup.py`
- [ ] Remove `setup.py` once the above is verified

## Verification

- [ ] `python -m build` produces an sdist + wheel byte-identical in contents (modulo metadata format) to the current release
- [ ] `pip install sentry-sdk[flask]`, `[django]`, `[opentelemetry]`, etc. resolve identically
- [ ] `py.typed` is shipped in the wheel
- [ ] `tests/` is **not** shipped in the wheel
- [ ] OTel propagator entry point is discoverable post-install
- [ ] Release pipeline dry-run succeeds

## Risks

- Public-facing extras names — a typo silently breaks `pip install sentry-sdk[name]`.
- Version bump automation — release tooling likely greps `setup.py`.
- `python_requires=\">=3.6\"` in current `setup.py` looks stale vs. the rest of the repo; worth confirming the intended floor before carrying it forward.

## Related

- Tox/uv migration: #6325, #6327, #6367, et al.
- #6121 (removed `bdist_wheel` universal option)
- #5932 (license classifier inconsistency — pyproject migration is a chance to fix cleanly via `license` field)