[0pt] Clarify tarantool update process to a newer major version

[Totktonada]

Our guarantee is that Nth major version can be upgraded to (N+1)th one, but we does not guarantee the same for (N+2)th version.

Say, to update 1.7 to 2.1 we need to update it first to 1.9, then to 1.10 and only then to 2.1.

We should clarify also what is a major version of tarantool: 1.7, 1.9, 1.10, 2. The way to enumarate versions was changed since 2.*.

[lenkis]

we need to revise the entire release policy doc then: https://www.tarantool.io/en/doc/2.1/dev_guide/release_management/#release-policy

[printercu]

Actually upgrading to 1.9 is not required, 1.7 -> 1.10 -> 2.1 works fine.

It's quite easy to migrate using docker

$ docker run --volume /path/to/workdir:/tnt -it tarantool/tarantool:1 sh
  sh> tarantool
    tnt> box.cfg{work_dir = '/tnt'}
    tnt> box.snapshot()
$ docker run --volume /path/to/workdir:/tnt -it tarantool/tarantool:2.1 sh
  sh> tarantool
    tnt> box.cfg{work_dir = '/tnt'}
    tnt> box.snapshot()

[Totktonada]

I would add to the Max's message: using box.snapshot() is important here. Otherwise you can receive the following errors:

E> ER_ACCESS_DENIED: Usage access to universe '' is denied for user 'admin'
F> can't initialize storage: Usage access to universe '' is denied for user 'admin'

[Totktonada]

The issue related to ER_ACCESS_DENIED: tarantool/tarantool#4606.