Update credentials to include Tenor API key

[ScoutHarris]

Ref #13803

This updates credentials files to include the Tenor API key from the secrets repo.

To test:

Verify the key is generated:

• In the project directory, go to Derived Sources. In ApiCredential.m, verify there is an entry for tenorApiKey.

Verify GIF searching:

• Go to Media > Add > Free GIF Library.
• Enter a search term.
• Verify search results are displayed.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[jkmassel]

✅ In the project directory, go to Derived Sources. In ApiCredential.m, verify there is an entry for tenorApiKey.
✅ Verify search results are displayed.

[ScoutHarris]

Thanks @jkmassel !