update readme for q2-2022.8

[mikerobeson]

This PR updates the README:

• Minimum version of QIIME 2 is now 2022.8
• Added link to the new Using RESCRIPt's 'extract-seq-segments' to extract reference sequences without PCR primer pairs. tutorial.
• Added link to the related db-benchmarks-2020 repo.

[mikerobeson]

I'm not sure why the build is failing now. Any thoughts?