fix: mutating ManyArray should handle duplicates gracefully (with deprecation)

[gitKrystan]

Description

Forward-port of #9126 (see #9119)

Re-enables deduplication of duplicate records added to ManyArrays, but with a deprecation (ember-data:deprecate-many-array-duplicates) as described below:

/**
 * **id: ember-data:deprecate-many-array-duplicates**
 *
 * When the flag is `true` (default), adding duplicate records to a `ManyArray`
 * is deprecated in non-production environments. In production environments,
 * duplicate records added to a `ManyArray` will be deduped and no error will
 * be thrown.
 *
 * When the flag is `false`, an error will be thrown when duplicates are added.
 *
 * @property DEPRECATE_MANY_ARRAY_DUPLICATES
 * @since 5.3
 * @until 6.0
 * @public
 */

Notes for the release