[RFE] Fully adopt new branching for BRO

[mallardduck]

Per title - this issue is to track the adoption of the new branching process.

Overview

This repo is already very close to the new standard, just missing a non-stalemain/master branch.
Currently, we have branches that match with Rancher versions like:

• release/v5.0 for 2.9; producing tags matching the branch,
• release/v4.0 for 2.8; same as above,
• release/v3.0 for 2.7; same,
• Other stale release branches for EOL ranchers left for historic reasons.

In our effort to adopt the new branching we will end up using main as our head branch.

To accomplish this goal we will end up with:

• main for head dev; producing a dev/latest/head tagged image,
• release/v5.0 for 2.9; producing tags matching the branch,
• release/v4.0 for 2.8; same as above,
• release/v3.0 for 2.7; same.

Specific Concerns

• Verify that CI workflows will work correctly for regular release tags and for head branch rolling dev tag,
• Need to make sure all recent CI changes are synced between master and active release/v* branches,
• Update the readme and docs as necessary,
• Create and push new main from release/v5.0,
• Ask EIO (or whomever responsible) to change default branch
• Update repo settings or verify settings and CI on new branch.

[mallardduck]

First step ready for review in this PR: #553

[mallardduck]

all recent CI changes are synced

Currently there is a slight divergence between (what will be) main and old ones. I'm comfortable with this as those shouldn't have any actual impact on the main branch.

[mallardduck]

We've just gotten the default branch updated to use main now.
I also bumped up the required reviews to 2 on release/* branches.
Soon we will be ready to enable required CI checks too - I think we have a few more ci tweaks before that though.