Modernize build versioning

[PTNobel]

Description

Modernizes the build/versioning setup along the same lines as cvxpy/cvxpy#3331:

• derive package versions from git tags with setuptools-scm
• generate diffcp._version at build time and re-export it as diffcp.version
• remove the stale .bumpversion.cfg release/version bump workflow
• fetch full git history in CI checkout steps so tags are available to setuptools-scm
• replace the stale setup.py develop Makefile target with a uv workflow
• move cvxpy out of runtime dependencies and into optional/test dependencies to avoid the circular dependency
• declare direct runtime dependencies (numpy, scipy, scs, threadpoolctl) instead of relying on cvxpy transitively
• explicitly compile the C++ extension as C++11 to avoid depending on newer compiler defaults

Verification

• uv build
• uv run python -c 'import diffcp; print(diffcp.version)'
• uv run --extra test pytest tests/ -q (43 passed, 2 warnings)