Deprecate convert_based_near_to_attributes in favor of nested_attributes flag

[laritakr]

Summary

Bulkrax::ValkyrieObjectFactory#convert_based_near_to_attributes was added before the generalized nested_attributes: true mapping flag existed. With the flag in place (#1193), the hardcoded translator can be retired in favor of a declarative mapping option.

Proposed steps

1. Add a deprecation warning to convert_based_near_to_attributes that fires when the translator actually runs and points users at the flag: "based_near translation via the hardcoded factory method is deprecated; declare nested_attributes: true in your based_near field mapping. This translator will be removed in Bulkrax 10.0."
2. Update Bulkrax-shipped default mappings (and any test app/fixtures) to include nested_attributes: true on based_near so they stop tripping the warning.
3. Document the migration path in the README/wiki: any custom Bulkrax mapping for based_near should add nested_attributes: true ahead of the next major release.
4. In the next major Bulkrax release, remove convert_based_near_to_attributes and the call site in transform_attributes.

Why now (a follow-up, not part of #1193)

The PR introducing the flag is intentionally scoped to "add the flag" so it can land cleanly. Migrating based_near is a behavior change for any adopter whose mapping doesn't yet declare the flag — keeping it separate gives that change its own discussion and release cadence.

Acceptance criteria

• Deprecation warning fires when the translator translates non-empty values, naming the flag.
• Bulkrax's own default mappings declare nested_attributes: true for based_near.
• Migration note in the README or wiki.
• Tracking ticket for the eventual removal in Bulkrax 10.0.