We already deprecated and removed the camel-case versions of the nGram and edgeNGram
filters a while ago and we should do the same with the nGram and edgeNGram tokenizers.
This PR deprecates the use of these names in favour of ngram and edge_ngram in 7
and disallows usage in new indices starting with 8.
Closes#50561
The removal of the "nGram" and "edgeNGram" token filter was filed in a new
migrate_8_0/mappings.asciidoc document but it should better be in an
analysis.asciidoc section and file.