Find out and list what was changed since 3.x.x -> 5.x.x and what we should remove or start to support

[visortelle]

Some candidates:

• Topic policies
• Namespace policies
• Resource metrics (for topics, subscriptions, producers, cursors, etc.)

We should support the current LTS and the lates available version of Pulsar at the same time.
Pulsar itself rarely introduces client breaking changes, so we can use the latest client versions.

At this moment it's 4.x.x and 5.x.x

We need the test env for this and a centralized list in code of such feature differences, so we could maintain it and conditionally enable/disable different features.