Update to ytdl-commit-6508688

Update to ytdl-commit-6508688

Make default upload_/release_date a compat_str
https://github.com/ytdl-org/youtube-dl/commit/6508688e88c83bb811653083db9351702cd39a6a

Except:
* "[NDR] Overhaul NDR and NJoy extractors" https://github.com/ytdl-org/youtube-dl/pull/30531
    - https://github.com/ytdl-org/youtube-dl/commit/01824d275bfa7efbaca274b38c1ddc2b03f12f5d
    - https://github.com/ytdl-org/youtube-dl/commit/39a98b09a2acf50dc64bc41185be723b98e740b9
    - https://github.com/ytdl-org/youtube-dl/commit/f0a05a55c2ee512880546c056cfbec5ad3399798
    - https://github.com/ytdl-org/youtube-dl/commit/4186e817772d49d6f66b07c5ac8c248f026a6446