docs: Update version number from 5.x to 6.x in README.rst

A quick 'grep "5\.x" . -R' on Documentation shows that README.rst,
2.Process.rst and applying-patches.rst all mention the version number "5.x"
for kernel releases.

As the next release will be version 6.0, updating the version number to 6.x
in README.rst seems reasonable.

The description in 2.Process.rst is just a description of recent kernel
releases, it was last updated in the beginning of 2020, and can be
revisited at any time on a regular basis, independent of changing the
version number from 5 to 6. So, there is no need to update this document
now when transitioning from 5.x to 6.x numbering.

The document applying-patches.rst is probably obsolete for most users
anyway, a reader will sufficiently well understand the steps, even it
mentions version 5 rather than version 6. So, do not update that to a
version 6.x numbering scheme.

Update version number from 5.x to 6.x in README.rst only.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20220824080836.23087-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>