Use the more modern English stemmer

[AA-Turner]

Purpose

Per #13535, the Porter stemmer is considered frozen, whereas the English stemmer still recieves improvements.

References

• English stemming problems #13535
• Update stemming and Snowball #13561
• Update URLs for the Snowball project  #13571
• Pre-parse stopword lists #13572
• Update language stemmer JavaScript to Snowball 3.0.1 #13573

[jayaddison]

NB: A dated dutch_porter-stemmer was also migrated to legacy status with Snowball v3.0.0 -- however no migration is required for Sphinx -- dutch refers to the updated, non-Porter version, and that's what we're using already.