feat: use base argument instead of series

[dimaqq]

Description

• support both 6 and 7 versions of Charms facade.
• deprecate the series= argument
• [to do] introduce base= argument

QA Steps

• [to do] validate using existing integration tests

Fixes #1156

[dimaqq]

@SimonRichardson early preview: a way to support Juju 3.6.

In a nutshell:

• use Facade v6 for old Juju (3.0 ~ 3.2.x)
  • exact same code path as before
• use Facade v7 for new Juju (3.3.0 ~ latest)
  • drop client-side series computation, let the controller decide
  • to do: handle explicit base
• deprecate series= kwarg

Major to do: support charms' existing integration tests against Juju 3.3.0 ~ 3.5.4

• convert series= to base=
• propagate this or explicit base=

[Gmerold]

Hi @dimaqq,
Do you plan to pick up this PR? It looks like its intent is exactly what my team needs for the tests to start passing ;)
Thanks!

[gatici]

Hello @dimaqq,
We are having a problem similar to #1240. Currently, it is a blocker for us.
I assume this PR will fix this issues. We appreciate if you could complete it as soon as possible.
Many thanks!

[dimaqq]

I think so, either I or someone from juju. Realistically in January though.

[dimaqq]

P.S. this PR is currently very raw. It doesn’t yet solve anything. The high level direction is clear though.