Update dependency on js package to 0.7.x

[john-paradym]

Summary

The js dependency in pub spec.yaml is using an old version of the js package

Even the 4.0.0 beta is using js 0.6.3, which is over 3 years old now

Motivations

We have other packages in our app that already have updated to js 0.7.1, but I cannot update them (without hacking a dependency override) until Amplitude updates to the later versions of js

[izaaz]

Thank you for the feedback @john-paradym. We haven't added browser support for the v4.0 SDK yet (it still contains the old browser support from the main branch). But i'll add a note to that feature to make sure this gets updated.