Update synthetic source documentation to state it's GA

[jdcryans]

Documentation like this https://www.elastic.co/docs/reference/elasticsearch/mapping-reference/text#text-synthetic-source has a note that synthetic source is only GA for TSDB. This is incorrect, as of 8.17 it's simply GA so the whole note should probably be removed.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[marciw]

hi @jdcryans @jordan-powers you can assign this to me -- thanks for flagging