The transition from swconfig to DSA is ongoing, but not yet complete. The process started with 21.02 and has added more targets/devices with each subsequent release, but there are more to go. Your device still uses swconfig. This doesn't present any problems though -- it works just as swconfig always has.
I seem to recall that the bridge inside the network interface config stanza was valid through 18.06, but deprecated for 19.07 and newer.
I don't know the specific reasons, but I would guess that the goal was to make things cleaner. If you start with the premise that each network has one virtual 'port' -- that is to say, one connection from the logical network configuration to hardware somewhere. If you want to be able to connect multiple physical devices[1], you need to use a bridge, which is basically an unmanaged switch implemented in software. By creating a separate bridge device stanza, we can group everyting in the bridge as a single 'device'. And then that single bridge device entity is used by the network interface.
Glad I was able to help get everything working!
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Thanks! 
with swconfig, the physical ethernet ports are handled by swconfig, but a bridge is necessary if you will be using two or more wifi radios, or ethernet + wifi. For devices that use DSA, the ethernet ports are handled directly by name, so in this environment, a bridge is required if there will be 2 or more total physical interfaces (i.e. individual ethernet ports and/or radios). ↩︎