Revert "Bump stream version to 2.4"

Rationale: Although the format has technically changed, CrystFEL 0.9.1
and older do not read the "hdf5/" lines anyway.  The new "header" lines
are just noise, as far as they are concerned.  However, increasing the
version number causes older CrystFEL versions to reject streams written
by newer CrystFEL versions.  This is inconvenient, but will be a
particular problem since CrystFEL 0.10.0 will most likely be released
without a working "geoptimiser".  We therefore need to be able to use
older CrystFEL versions for refining geometry.  Older versions are
perfectly capable of reading newer streams.

In general, the semantics of stream version numbers are a bit messed up,
and we need to work out what they really mean.

This reverts commit fdce01adf9fc5950904b644ea755760cb32ec5b2.

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