Plan features for 1.2.x

Problem/Motivation

HTMX has a major version update so that should be a minor version change.

Let's add other features below.

Proposed features

• #3456519: Move to HTMX v2.x
• #3456435: Implement HtmxAttributes class per #3456070: [policy, no patch] Choose a markup strategy for HTMX POC
• #3459112: Refactor asset and behaviors management away from swap-oob

Remaining tasks

User interface changes

API changes

Data model changes