The big cleanup to get vuetify v3 as stable as v2

[stratdev3]

I'm aware of the context the author is facing : not enought sponsorship to get paid and live for and the pressure of users waiting the final version.

So, while still suffering from some reported issues, some with a related PR not merged, and a v2/v3 mixed documentation, vuetify v3.0 was release today.

By promoting the beta-15 as a final without RC, maybe the plan is to buy extra time : while users upgrade their codebase to v3, maintainers will fix all the problems in a v3.0.x.

This move will work better after clean up the 1050 opened Issues and 90 waiting PR. Because it's quite frankly complicated when trying to help on the project.

Some direction to explore :

1. opened issues before 2021 : close them because they are not relevant anymore. Either the opener has not followed is own issue or maintainer let it die.
2. handle PR : developer spend time to find, understand and propose a fix to help. refuse or accept ASAP.
3. v2 is strong and stable, all efforts should be made to v3 because is the next BIG thing with vue3/vite.

At last, thanks to all of whose working of this awesome project