fix: drop support for Vite 2

[antfu]

Given the fact that Vite 3 is out for about two months and most of the ecosystem is finished upgrading, we could drop the compact range for Vite to allow non-vite usage always to install Vite 3 as the dependency.

[dammy001]

@antfu we might need to update the version on docs as well

[alexjidras]

@antfu will we still be able to use vitest with vite 2 ?

[sheremet-va]

@antfu will we still be able to use vitest with vite 2 ?

Technically Vitest will use your vite config, but will run in through Vite 3, so it will depend on your plugins mostly.