Docs: Replace version by 'current' in changes.rst

The file 'Documentation/process/changes.rst' states the listed
requirements are for the 4.x kernel version. However, there are
requirements updated for the 5.x version, as there might be in other
future versions. This patch updates it to 'latest' so the document won't
be outdated in the future.

Signed-off-by: Bruno Moreira-Guedes <codeagain@codeagain.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>