refactor(billingAddons): normalize NLP usage naming TASK-1015

[jamesrkiger]

Checklist

1. If you've added code that should be tested, add tests
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Description

Previously we were using 'nlp_character_limit' and 'nlp_minutes_limit' as the keys for nlp usage limits in Stripe metadata. Elsewhere in the code we have been using 'asr_minutes' and 'mt_characters'. This PR removes the Stripe-specific language for consistency. A separate PR will be submitted to update the frontend accordingly.

[notion-workspace]

Switch to mt_characters and asr_minutes in code/stripe