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
ddf189892c