Restrict default field name length

[henningandersen]

The default field name length limit is unbounded. This imposes a risk to cluster health in that if a client generates wildly long field names, the resulting mapping can become large, causing cluster wide issues.

We propose to lower the default limit of field names to 10K characters. This should be done in both 8.0 and 7.x, with an opt out option.

This should only apply to field mappings created after an upgrade to a version with the new default limit.

Related to #40309

[elasticmachine]

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

[markharwood]

We discussed this in the es-search meeting and had the following observations:

• 10k is much too large for a limit
• this limit should apply to new indices only
• it's unclear if the limit should be on individual JSON field names or on the cumulate full path name caused by nested objects. (Nesting has a limit of 20 objects so we are looking at max 20x multiplier)