Update 15.4 to 15.5 how-to re 4.6.1-0 compatibility

[phillxnet]

We have the following text at the head of Distribution update from 15.4 to 15.5: https://rockstor.com/docs/howtos/15-4_to_15-5.html

• A minimum Rockstor version of 5.0.9-0 is recommended before attempting this OS update.

Which is non-inclusive for Stable subscribed systems, yet an in-place update of a 4.6.1-0 stable install is possible, and has been tested. This warning was added predominantly to reference the readiness in testing channel only.

• Modify this message to include the stable version also, i.e.

A minimum Rockstor version of 4.6.1-0 (stable) or 5.0.15-0 (testing) is recommended before attempting this OS update.
Where we have also substituted the most recently available rpm version for 15.4 in the testing channel.

[phillxnet]

This change is also pertinent re availing folks of an update path to attain a base OS for which we intent to publish our next Stable rpm: 15.5. Without first having to switch to a testing subscription: which could, in time, carry the risk of accidentally installing an early testing rpm. At least once we are done with our current late testing phase; and have moved into early next testing phase.