chore(sdist): migrate `project.license` to SPDX expression

This pull request updates the project's license declaration in pyproject.toml to use an SPDX expression and refines build system metadata.

What was done

Migrated the project.license field in pyproject.toml from a dictionary with text = "Apache 2.0" to a direct SPDX expression string "Apache-2.0".
Added a license-files = ["LICENSE"] entry under the [project] table in pyproject.toml.
Removed the license-files = ["LICENSE"] entry from the [tool.setuptools] table to consolidate license file declaration.
Updated the minimum required version for setuptools in build-system.requires from >=61 to >=77.0.3.
Removed the redundant classifier License :: OSI Approved :: Apache Software License from the project's classifiers.

Improves compliance and standardization of license declaration using the SPDX format.
Ensures the build system uses a more recent and potentially more robust version of setuptools.
No direct user-facing changes or impact on runtime behavior.
Simplifies project metadata by consolidating and removing redundant license declarations.

The pyproject.toml file was modified extensively to update project metadata.
The project.license field now uses the SPDX short identifier "Apache-2.0".
The license-files field was moved to the [project] table, aligning with modern Python packaging standards.
The build-system.requires list was updated to enforce setuptools>=77.0.3.
A superfluous license classifier was removed as the SPDX identifier is now the primary source for license information.