fix(googleapis-common-protos): remove deprecated license classifier

[btschwertfeger]

This PR drops the deprecated classifier from the project metadata of googleapis-common-protos.

It also fixes the license to be SPDX-compliant which was also warned during the built of the project (https://spdx.org/licenses/):

********************************************************************************
Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

By 2027-Feb-18, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************

Closes #18235

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates the pyproject.toml file for googleapis-common-protos by simplifying the license field to a string value "Apache-2.0" and removing the "License :: OSI Approved :: Apache Software License" classifier. There are no review comments, and I have no feedback to provide.