Promotes ESQL search functions from preview to GA in v9.1.0 and adds multi-valued string support, cleanup

• upgraded several esql search functions (match, kql, qstr) from preview to ga in version 9.1.0
• updated applies_to annotations to specify both preview 9.0.0 and ga 9.1.0 lifecycle stages
• added version-specific documentation examples with applies_to markers for ga 9.1.0 features
• enhanced to_lower and to_upper functions to support multi-valued expressions in ga 9.1.0
• updated multi_match function from "coming" to preview 9.0.0 status
• removed preview markers from search function lists and json definitions
• fixed typo in json generation comment ("do no edit" → "do not edit")
• added version guards around function parameters and descriptions using applies_to syntax
• updated function parameter descriptions to indicate ga 9.1.0 availability for named parameters

Author: leemthompo