Deprecate main branch

[rootulp]

Context

What Cosmos SDK major release is main in this repo based on?

Still v0.46. It was cut from releases/v0.46.x-celestia. Actually I brought it up in slack that given we don't plan to break anything (i.e. we will use version based if statements) that it would be okay to just use the single branch and thus we don't need main

Originally posted by @cmwaters in #358 (comment)

Problem

This repo contains an undocumented main branch that we may not need or use

Proposal

Deprecate the main branch

Prerequisite

Get confirmation from @cmwaters that it's safe to deprecate and that nothing on main needs to be backported to the release branch

[rootulp]

Note a repo admin or maintainer w/ permissions needs to tackle this.

[cmwaters]

I'll port my upgrades change to the v0.46 branch and then I think we can deprecate it

[cmwaters]

Ref: #360