[FFmpegMetadata] Store release_year in the metadata

[unkernet]

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• [x] Searched the bugtracker for similar pull requests
• [x] Read adding new extractor tutorial
• [x] Read youtube-dl coding conventions and adjusted the code to meet them
• [x] Covered the code with tests (note that PRs without tests will be REJECTED)
• [x] Checked the code with flake8

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What is the purpose of your pull request?

• [ ] Bug fix
• [x] Improvement
• [ ] New extractor
• [ ] New feature

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Description of your pull request and other information

Many music extractors returns only release_year field, and it is not stored in the metadata.