Constants for TransportVersion currently live alongeside the class
definition. This has been fine since there was only one set of
constants. However, to support serverless, some constants will need to
be defined elsewhere.
This commit moves the existing constants to a new holder class,
TransportVersions. It is almost entirely mechanical, using IntelliJ move
members. The only non mechanical part was slightly shifting how CURRENT
is found, defining a LATEST in TransportVersions that is automatically
calculated (since we already have it, no need to manually define it).
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