[Maintenance] Deprecate usage of META_FIELDS_BEFORE_7DOT8

[ykmr1224]

MapperService.META_FIELDS_BEFORE_7DOT8 is marked as deprecated and will be deleted. We need to migrate to MapperService.isMetadataField to continue distinguishing metadata fields.

MapperService.META_FIELDS_BEFORE_7DOT8 is used in legacy engine to distinguish metadata fields and document fields. Current usage refer the constants, but the constant is no longer correct since there could be more metadata fields used by other plugins.

[dblock]

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