[pp/FFmpegMetadata] Convert date strings to ISO 8601

[7x11x13]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

This PR converts the YYYYMMDD format used by extractors into YYYY-MM-DD format before embedding into the file with FFmpeg. It also uses release_date over upload_date as suggested: #12230 (comment)

Related:

• Wrong date format in opus/m4a tags #12230
• Mutagen metadata postprocessor #11817 (comment)

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

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)