message: Changelog: mention the shift to using cfg features
diff: - Renamed `filters::gzip_available()` to `deflate_available()` (the old name is
  present but marked as deprecated).
  +- Code dependent on HDF5 version in `hdf5` and `hdf5-sys` crates now uses features
  + instead of cfg options: `cfg(feature = "1.10.1")` instead of `cfg(hdf5_1_10_1)`.
  + The main initial reason for that is for HDF5 versions to show up in the official
  + documentation on docs.rs.
  +- Similar to the above, there's `have-direct`, `have-parallel` and `have-threadsafe`
  + features that reflect the build configuration of the underlying HDF5 library.
  ### Fixed
  