Add presubmits for checking constraints files using `lower-bound-checker` from `google-cloud-testutils`

[parthea]

Similar to https://github.com/googleapis/gapic-generator-python/issues/1870, there should be a presubmit to confirm that the minimum supported version of a dependency in setup.py is being tested via the constraints files.

See https://github.com/googleapis/synthtool/issues/1901 for context.

The check_lower_bounds nox session exists in gapic-generator-python but not in synthtool templates here.