proto-plus >= 1.25 needed for python 3.13

[parthea]

The setup.py file should be updated to require proto-plus >= 1.25 for python_version >=3.13 due to googleapis/proto-plus-python#490

See release notes here: https://github.com/googleapis/proto-plus-python/releases/tag/v1.25.0

Add "proto-plus >= 1.25.0, <2.0.0dev; python_version>='3.13'", after the following line

python-documentai-toolbox/setup.py

Line 60 in d82f689

"proto-plus>=1.22.3, <2.0.0dev",

As an example, see googleapis/python-api-core#740