[BUG] docs: "SaltStack git Policy" still references old branching/release model

[eliasp]

Description
The current state of "SaltStack git Policy" in the docs still references the old pre-3000 branching/release model.

This needs to be updated accordingly to match e.g.:

• https://docs.saltstack.com/en/master/topics/development/contributing.html#salt-s-branch-topology
• https://docs.saltstack.com/en/master/topics/development/conventions/release.html

[sagetherage]

@saltstack/docs-working-group this is going to be needed for Sodium release, let me know if the WG can't review prior to the feature freeze date - which has been influx but in the next week.

[sagetherage]

@barbaricyawps @ScriptAutomate saltstack/salt-enhancement-proposals#20 outlining the new branching strategy and here are paths the rst files referenced above:

salt/doc/topics/development/git/index.rst
salt/doc/topics/development/conventions/releases.rst

We may want to look for more references to branching and releasing in the future.

[sagetherage]

@eliasp we also accept PRs for review! Thank you for creating an issue.

[sagetherage]

@ScriptAutomate this is smaller in scope, and lingering change to the docs.