Lower js constraint to allow 0.6.x

[thedalelakes]

Because google_maps_flutter >=2.6.0 depends on google_maps_flutter_web ^0.5.6 which depends on google_maps ^7.1.0, google_maps_flutter
  >=2.6.0 requires google_maps ^7.1.0.
And because google_maps >=6.3.0 depends on js ^0.6.3 and mixpanel_flutter >=2.3.0 depends on js ^0.7.1, google_maps_flutter >=2.6.0 is
  incompatible with mixpanel_flutter >=2.3.0.
So, because <app_name> depends on both google_maps_flutter ^2.6.0 and mixpanel_flutter ^2.3.0, version solving failed.

I'm on the latest version of both google_maps_flutter and mixpanel_flutter. Upgrading the js dep to only ^0.7.x broke compatibility with google_maps_flutter, and I'm sure other plugins were affected as well.

This PR lowers the constraint to allow 0.6.x.

[zihejia]

Thanks @thedalelakes @cerberodev !

[chadpav]

I have this conflict with a couple of packages as well. Can you cut a release with this change in it? It's merged into main but that came after the 2.3.1 release.