ESQL: Hide spatial grid functions behind SNAPSHOT (elastic#129839)

elastic#125143 added 9 spatial grid functions and released them into Serverless. We think this is not the best long-term approach and the functions in elastic#129581 are likely better.

As a first step, rmove the spatial grid functions added in elastic#125143 from release builds so they don't get released into 8.19/9.1.

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Co-authored-by: Craig Taverner <[email protected]>
(cherry picked from commit efb1397)

# Conflicts:
#	docs/reference/query-languages/esql/_snippets/lists/spatial-functions.md
#	docs/reference/query-languages/esql/functions-operators/spatial-functions.md

Author: alex-spies