Fix API endpoints for related media and provider stats

[zackkrida]

Fixes

This PR fixes the endpoints to fetch related media, updates to use the new endpoints for provider stats, and removes fallback default provider data that was hardcoded into the repo, which can give false assurances that things are working.

We'll need to test API changes on the front-end much more closely moving forward.

Noting that in production, the related API endpoint is still seemingly broken, although my local instance of the API is working.

Description

Some additional low-hanging cleanup here:

• Fix audio endpoints from 'audios' to 'audio'
• Remove unused provider collection endpoints
• Add some jsdoc annotations for axios
• Fix error handling of getProviderStats, which wouldn't actually catch exceptions
• Start the work to make the Sources page media-type agnostic (it still doesn't actually fetch the audio providers)

Testing Instructions

• Run locally and observe the provider list on the 'sources' page and in the results filter sidebar is visible.
• Run tests and ensure they work
• Bonus: Run a local instance of the API and change the nuxt.config.js to point to it (don't forget v1 at the end of the endpoint!). Confirm that locally configured sources are visible.

Checklist

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• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[sarayourfriend]

LGTM, tests great. Hopefully we can figure out why related images are broken but at least the critical path is fixed.

[AetherUnbound]

Looks good!

[obulat]

I tried it with the local API, and everything works great!
I added code suggestions to remove a stray log statement and a small change.

[obulat]

I get the 400 error for related endpoint, it is caused by pagination error in the API:
https://github.com/WordPress/openverse-api/blob/13673ee8c196c9a6b786a1322d6d65b97c6ca708/openverse_api/catalog/api/utils/pagination.py#L29