Revise upgrading instructions in READMEs

[pkhartsk]

In version-specific READMEs:

This description seems very outdated and I'm not sure that's how things actually work anymore, since (even as shown by the attached link) one can upgrade through multiple Y versions at the same time. This description should be revised.

MySQL and MariaDB use versions that consist of three numbers X.Y.Z (e.g. 5.6.23).
For version changes in Z part, the server's binary data format stays compatible and thus no
special upgrade procedure is needed. For upgrades from X.Y to X.Y+1, consider doing manual
steps as described at
https://mariadb.com/kb/en/library/upgrading-from-mariadb-104-to-mariadb-1011/
Skipping versions like from X.Y to X.Y+2 or downgrading to lower version is not supported;
the only exception is ugrading from MariaDB 5.5 to MariaDB 10.0 and from MariaDB 10.3 to 10.11.