This commit renames enums related the "Curve" object type and ID type
to add `_LEGACY` to the end. The idea is to make our aspirations clearer
in the code and to avoid ambiguities between `CURVE` and `CURVES`.

Ref T95355

To summarize for the record, the plans are:
- In the short/medium term, replace the `Curve` object data type with
 `Curves`
- In the longer term (no immediate plans), use a proper data block for
  3D text and surfaces.

Differential Revision: https://developer.blender.org/D14114