Add support for deprecated names, raise warning instead of error for unknown filter values, refactor descriptors code.

[aaronweeden]

This PR depends on [upcoming PR in xdmod] that will need to be merged first for the CI tests to pass on this one.

Description

This PR does the following:

• Add support for deprecated names (in conjunction with an upcoming PR to xdmod), where the XDMoD portal can mark names of realms, metrics, dimensions, and/or raw fields as deprecated, and xdmod-data will throw a FutureWarning if it is used while using the updated name for its requests to the portal.
• Change to raising a warning instead of an error when a provided filter value is not found.
• Refactor the descriptors code (doesn't change the API):
  • Rather than a single object containing both descriptors, have one object each for aggregate and raw descriptors, each with their own specialized class.
  • Finding IDs and labels in descriptors is now done in the descriptor classes instead of in functions in other classes (to encapsulate behavior).
• Update the maximum Python version that is tested in the CI build.

Motivation and Context

See [upcoming PR in xdmod].

Tests performed

This PR updates the integration tests to make sure:

• The FutureWarning is properly raised for a deprecated raw field.
• Unknown filter values produce a UserWarning instead of a KeyError.

Type of change:

• Refactoring / documentation update (non-breaking change which does not change functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix, feature, or removal that would cause existing functionality to change)
• Release preparation

Checklist:

• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• Updates have been made to the xdmod-notebooks repository as necessary, and the notebooks all run successfully