Consolidate setup.py boilerplate to a single source at the monorepo root.

### Summary of the feature request

Consolidate setup.py boilerplate to a single source at the monorepo root.

**Current State**: 26 packages define this file.
**Similarity Profile**: 53.6% similarity, with a mean of 99 lines of code.

**Why it's a prime candidate**: A legacy boilerplate file containing package descriptors, classifiers, and python version constraints.

**Recommendation**:
Migrate package configurations to modern, declarative `pyproject.toml` files utilizing PEP 621.
Use a centralized build system script or setup generator (such as our `gapic-generator` templates) to cleanly manage and inherit common metadata attributes (like licensing, Python version classifiers, and author info) from a root-level configuration.