Remove null-safety from pub.dev package versions

[dcharkes]

The newer Dart SDKs are not supporting non-null-safe code anymore, but our UI still displays null-safety status of packages:

By this point it's just UI clutter, so I propose we remove it.

cc @sigurdm

[sigurdm]

@isoos this should be a simple change right?

@mit-mit do we have anything we want to replace this with? wasm-readyness perhaps?
Or just yank it out?

[mit-mit]

@kevmoo can you comment on the wasm part? I'm not aware of anything else besides that.

[kevmoo]

I'd love to add wasm-readiness to it. But we need to evaluate false positives/negatives.

[sigurdm]

Ok, let's decouple.

For now let's just remove the null-safety tag. And then we can add something else later if we want.

[isoos]

I've found #6463 as a proposed replacement (Dart3-compatible), but it requires an analysis run, and we don't do it for all versions.