Downgrading the 'datasets' package from 3.0.0 to 2.21.0 in some notebooks of tutorials

# What does this PR do ?

'[datasets](https://pypi.org/project/datasets/#history)' recently released the latest version 3.0.0 on Sep 11th, but some HF datasets (e.g., [MCV 3.0](https://huggingface.co/datasets/mozilla-foundation/common_voice_3_0), [MCV 6.1](https://huggingface.co/datasets/mozilla-foundation/common_voice_6_1)) do not update their code for the latest version. If we try to download these MCV with the latest 'datasets', it will raise an error: _TypeError: DownloadConfig.__init__() got an unexpected keyword argument 'ignore_url_params'._ 
Hence, we need to downgrade the 'datasets' package to the last version 2.21.0 in some tutorial notebooks. Please check the first block of each notebook.

**Collection**: [Note which collection this PR will affect]
NeMo/tutorial/asr 

# Changelog 
- Add specific line by line info of high level changes in this PR.
Updated requirements file 

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