Removes old Lucene's experimental flag from analyzer documentations

[jimczi]

This change removes the Lucene's experimental flag from the documentations of the following
tokenizer/filters:

• Simple Pattern Split Tokenizer
• Simple Pattern tokenizer
• Flatten Graph Token Filter
• Word Delimiter Graph Token Filter

The flag is still present in Lucene codebase but we're fully supporting these tokenizers/filters
in ES for a long time now so the docs flag is misleading.

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[elasticmachine]

Pinging @elastic/es-docs (:Docs)

[elasticmachine]

Pinging @elastic/es-search (:Search/Analysis)

[romseygeek]

LGTM

[jimczi]

Thanks @romseygeek and @jrodewig