Rename verbose plural "mature" to "sensitive" in API admin

[AetherUnbound]

Description

Quick follow up to #3769, when looking in staging I realized that there was one instance of "mature" that I hadn't updated 😅

This is a tiny PR to update the last reference!

There's a migration attached to it but it's "in-Django" only:

$ j api/dj sqlmigrate api 0057

BEGIN;
--
-- Change Meta options on sensitiveaudio
--
-- (no-op)
COMMIT;

Testing Instructions

1. Build and run the app locally
2. Visit http://localhost:50270/admin/api/sensitiveaudio/ and ensure that the text on the side bar says "Sensitive audio" and note "Mature audio"

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[github-actions]

This PR has migrations. Please rebase it before merging to ensure that conflicting migrations are not introduced.

[dhruvkb]

LGTM, thankfully no DB migration.