[AdultSwim] fix ignoring subtitles warning

[nnoboa]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

Fixes #6191

This PR fixes a warning message stating "subtitle tracks found in the HLS manifest" are being ignored, following the change suggested in the issue.

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

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• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
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Copilot Summary

🤖 Generated by Copilot at 47fdc67

Summary

🎞️🗣️🔀

Improve subtitle extraction and merging for adultswim extractor. Use m3u8 source to get subtitles and combine them with existing ones.

Extract m3u8 formats
Subtitles join the info dict
Autumn of merging

Walkthrough

• Extract subtitles from m3u8 formats and merge them with existing subtitles (link)